Skin-to-deltoid-muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study
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Messenger ribonucleic acid (mRNA) vaccines developed to protect against SARS-CoV-2 infection are only approved for intramuscular administration.[[1,2]] A needle of sufficient length to penetrate through the skin and subcutaneous tissue is needed to reach the deltoid muscle (the skin-to-deltoid-muscle distance), which is the preferred injection site.[[3]] It has been suggested that penetration of the deltoid muscle by 5 millimetres (mm) is required to ensure intramuscular deposition of the vaccine.[[4]] With national rates of adults with obesity above 40% in the United States of America (USA),[[5]] and around a third in Australia[[6]] and New Zealand,[[7]] it is important to choose an appropriate needle length for this demographic. Published data report that a standard needle length (25mm) is suitable for most people with a body mass index (BMI) <25 kilograms [kg]/m2.[[4,8]] However, progressively higher BMIs increase the likelihood of requiring a longer-than-standard needle length for deltoid intramuscular injection.[[8–10]]
Worldwide, immunisation guidelines vary in their instructions on how to choose the correct needle length based on BMI and body weight, or contain non-specific terms such as “larger arms”.[[11–13]] An accurate measurement of BMI for a vaccine recipient is not always readily available and the interpretation of arm size is subjective, resulting in an increased risk of inappropriate needle length choice and subcutaneous vaccine delivery. An observational study of a SARS-CoV-2 mRNA vaccine, administered with needles of different lengths at the discretion of the vaccinator, did not demonstrate a difference in immunogenicity between those vaccinated with a needle of sufficient versus insufficient length to achieve intramuscular deposition of vaccine.[[14]] However, there is evidence that intramuscular injection results in significantly better immune response compared to subcutaneous delivery of influenza and hepatitis B vaccines.[[15]] Further, there is high-grade evidence that subcutaneous administration of different vaccine types (adjuvanted, live virus and non-adjuvanted) is associated with increased local side effects including abscess and granuloma formation, compared to intramuscular delivery.[[15–17]]
The location of the deltoid intramuscular injection site is defined variably between countries based on anatomical landmarks (Figure 1). In the USA, the recommended injection site is 2 inches (approximately 5 centimetres [cm]) below the acromion process and above the axillary fold.[[13]] In Australia, the site recommended for injection is the midpoint between the acromion process and the deltoid tuberosity.[[11]] In New Zealand, the site recommended is at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting the acromion and the deltoid tuberosity.[[18]]
Current guidance on needle length choice does not account for potential variation in skin-to-deltoid-muscle distance across the length of the deltoid muscle. In two studies,[[19,20]] a statistically significant difference in mean skin-to-deltoid-muscle distance between injection sites was reported in individuals with a low/normal BMI of approximately 20kg/m[[2]]. With the relatively small skin-to-deltoid-muscle distances in this population, the absolute differences were not clinically important for intramuscular injection needle length recommendations. Whether the same variability in skin-to-deltoid-muscle distance exists in a population with obesity is unknown, yet highly relevant since they are at risk of subcutaneous injection from insufficient needle length. If within-subject differences in skin-to-deltoid-muscle distance at larger BMIs are of the same relative magnitudes to that reported by Nakajima et al.,[[18,19]] the injection site chosen may alter the required needle length. It is therefore important to understand in adults with obesity the difference in skin-to-deltoid-muscle distance between recommended injection sites worldwide to help inform correct choice of needle length.
The objective of this study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by national guidelines in the USA, Australia and New Zealand. The study also aimed to explore if any identified differences in skin-to-deltoid-muscle distance between the three recommended injection sites differed by sex, BMI or arm circumference.
Subjects
Eligible participants were 18 years or older, of any ethnicity, with a BMI of 30kg/m[[2]] or more, who provided informed consent prior to participation. Recruitment took place by direct invitation of potential participants on the Medical Research Institute of New Zealand (MRINZ) database and by advertisement on social media. No stopping criteria applied, provided participants did not withdraw consent before completion of their study visit.
Materials and methods
This was a single-site non-interventional cross-sectional study conducted at the MRINZ in Wellington, New Zealand. All investigations were completed in a single visit of approximately 45 minutes after obtaining informed consent.
Participant date of birth was used to determine age. Participant self-reported ethnicity, side of non-dominant arm and comorbidities were recorded. Ethnicity is reported as prioritised ethnicity using Level 1 codes, a method of reducing multiple ethnicities for analysis in the health and disability sector, which is used in the Statistics New Zealand Census.[[21]] Participant height and weight were measured by using a calibrated stadiometer and body scale (BWB-800, Wedderburn, New Zealand). Derived BMI was calculated.
Participants were instructed to expose their non-dominant arm and hang it relaxed by their side. First, the acromion process was identified by palpation and a mark was placed with an indelible pen (point A), followed by a mark on the skin corresponding with the deltoid tuberosity (point D). The USA injection site (point USA) was identified and marked by measuring 5cm inferior to point A, followed by the Australian injection site (point AUS) at the exact midpoint between the point A and point D, and the New Zealand injection site (point NZ) at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting point A and point D (Figure 1). All markings were checked by a second investigator before the distances were measured (in cm) between all points.
For each participant, with the shoulder in anatomical position and elbow passively flexed and slightly pronated, three ultrasound images displaying the skin, subcutaneous tissue and fascia and the deltoid muscle were captured and saved using a high-frequency (13-6 MHz) linear transducer (Sonosite X-Porte, Fujifilm, Japan), after using sufficient water-soluble ultrasound transmission gel as an acoustic standoff. The middle of the ultrasound probe was placed at the marked injection site, with minimal pressure, at a 90-degree angle with the skin, in the coronal plane. Penetration depth setting was increased as required to ensure a sufficient volume of the deltoid muscle was displayed. Ultrasound images were obtained by trained clinical staff (LK, SK). Measurements of the distance (in mm, to the nearest whole mm) between the skin and the fascia of the deltoid muscle were performed by a radiology registrar (JB). For each participant, two consecutive measurements of arm circumference were performed at point USA, point AUS and point NZ with their shoulder in 90 degrees passive abduction.
The primary outcome was the difference in skin-to-deltoid-muscle-distance (in millimetres) between three different sites recommended for intramuscular vaccination. The secondary outcomes included i) associations between skin-to-deltoid-muscle distance across the three recommended injection sites, with sex, BMI and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20mm, in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Sample size
The sample size was based on publications by Nakajima et al.,[[19,20]] with a view to detect a difference of 0.6mm with 80% power and a two-sided type I error rate of 5%, and further increased to allow for four explanatory degrees of freedom (distance from acromion, sex and arm circumference). An analysis was undertaken after recruitment of 40 participants to assess the size of the paired SD, which was sufficient to end recruitment at this point.
Data analysis
Continuous data were described by mean and standard deviation (SD), median and inter-quartile range (IQR) and minimum (min) to maximum (max). Proportions were described by counts and proportions expressed as percentages. On the scatter plot, linear regression lines are shown. The estimates of the associations between skin-to-deltoid muscle distance are by mixed linear models with fixed effects for measurement site, sex and arm circumference or BMI; together with associated interaction terms and, as random effects, the individual participants with unstructured variance-covariance correlation structures for the repeated measurements. The difference in paired proportions was estimated by appropriate categorical data models for the proportion of participants with skin-to-deltoid-muscle distances above nominated thresholds.
A threshold of p<0.05 was used to determine if between-site differences in mean skin-to-deltoid-muscle distance were statistically significant. The primary comparisons used the New Zealand definition of injection site as the reference level and compared this to the Australia and USA sites (Hypothesis 1). The secondary analyses included a one degree of freedom test using the ordinal rank of the distance from the acromion process as a predictor to explore if skin-to-deltoid-muscle distance increases as the distance increases (Hypothesis 2). To explore the possibility that the difference in skin-to-deltoid-muscle distance between sites differed by sex (Hypothesis 3), BMI (Hypothesis 4), and arm circumference (Hypothesis 5) were explored using mixed linear models with main effects and interaction terms for each of these possible effect modifying predictors. A comparison of the paired proportions was done by a generalised mixed linear model (Hypothesis 6). SAS version 9.4 was used for statistical analyses.
Ethical approval
This study was approved by the Northern B Health and Disability Ethics Committee (REF: 2022 EXP 12121). All participants provided written informed consent prior to participation.
Results
View Figures 1–2 and Tables 1–3.
There were 29 female and 11 male (n=40) participants with a mean (SD) age of 52.0 (16.3) years. There were no missing data. Demographic and anthropometric data are shown in Table 1. Mean (SD) BMI was 36.8 (5.0) kg/m[[2]] and the mean (SD) skin-to-deltoid-muscle distance for the USA, Australia and New Zealand sites were 13.96 (4.54), 17.94 (6.08) and 20.26 (5.91) mm respectively. Note that a recommended needle length is one that is 5mm greater than the skin-to-deltoid-muscle distance. Using prioritised ethnicity, 90% of participants were European, 5% were Māori, 2.5% were Pasifika, and 2.5% were of Middle Eastern/Latin American/African ethnicity. Respiratory disease was the most common comorbidity, being present in 70%, followed by cardiovascular disease in 27.5% and diabetes in 5% of the study population. This table also shows variability in the distances to injection sites in relation to anatomical landmarks recommended by the Australian and New Zealand guidelines and the summary data of the arm circumference measured at each injection site.
Differences in skin-to-deltoid muscle distance between recommended injection sites
The estimates of the differences between skin-to-deltoid-muscle distance ultrasound measurements between injection sites are displayed in Table 2. There was an interaction between site of measurement of skin-to-deltoid-muscle distance and sex, P-interaction 0.035; the sex-specific differences are also shown. The interpretation is that skin-to-deltoid-muscle distance is larger for the New Zealand than the Australia and USA sites and that these differences between injections sites are in turn larger for men compared to women.
Skin-to-deltoid-muscle distance in relation to anthropometric measurements
Distance from acromion
When the three measurement sites are treated as ordinal scale variables one unit apart in the order of increasing distance from the acromion (USA then Australia then New Zealand sites), the estimate of increase in skin-to-deltoid-muscle distance per “unit” increase in measurement site is 3.1mm (95% CI 2.7 to 3.6).
Arm circumference
Increasing arm circumference was associated with greater skin-to-deltoid-muscle distance for all three injection sites (Figure 2a and Figure 2b) independent of sex (p=0.56), measurement site (p=0.30), or the combination of sex and measurement site (p=0.39). The estimated coefficient (95% CI) was 0.77 (0.51 to 0.97) mm increase in skin-to-deltoid-muscle distance per 1cm increase in arm circumference.
BMI
There was a positive association between skin-to-deltoid-muscle distance and BMI for all three injection sites (Figure 3c and Figure 3d), independent of sex (p=0.33), or measurement site (p=0.27) or the combination of sex and measurement site (p=0.10). The estimated slope (95% CI) was 0.65 (0.40 to 0.90) mm increase in skin-to-deltoid-muscle distance per unit increase in BMI.
Proportions of participants with skin-to-deltoid-muscle distance greater than 20mm and 33mm
The two standard needle lengths in New Zealand are 25mm and 38mm. After allowing for 5mm penetration into the deltoid muscle these needle lengths correspond to skin-to-deltoid-muscle distances of 20mm and 33mm. No participant had a skin-to-deltoid-muscle distance of greater than 33mm at any measurement site, and so based on this sample no participant would require a needle length greater than 38mm. The counts and proportions of participants with a skin-to-deltoid-muscle distance greater than 20mm, and therefore needing a needle length of greater than 25mm, including cross-classification by measurement site, are shown in Table 3. These proportions were 45% for the New Zealand, 40% for the Australian and 15% for the USA measurement sites respectively. The differences in paired proportions (95% CI) were USA versus New Zealand, 30% (15.8 to 44.2), p<0.001, and Australia versus New Zealand, 5% (-1.8 to 11.8), p=0.15.
Discussion
This study has identified that in adults with obesity, defined as a BMI above 30kg/m[[2]], there are marked differences in the skin-to-deltoid-muscle distance measured at three recommended sites for deltoid intramuscular vaccine injection. The magnitude of the differences was of clinical importance, with the New Zealand site having a mean skin-to-deltoid-muscle distance approximately 6mm and 4mm greater than the recommended sites in USA and Australian guidance respectively. Skin-to-deltoid-muscle distance was greater in females, and participants with greater BMI and larger arm circumference were likely to have greater skin-to-deltoid-muscle distances. These findings suggest that when choosing the appropriate needle length to achieve intramuscular injection in a population with obesity, consideration needs to be given to the specific deltoid injection site selected, as well as the sex, BMI and/or the arm circumference of the individual.
The mean skin-to-deltoid-muscle distance (SD) was 14.0mm (4.5), 17.9mm (6.1) and 20.3mm (5.9) at the recommended USA, Australian and New Zealand injection site respectively. These measures can be considered in relation to the standard 25mm needle used for intramuscular injection of vaccine in clinical practice. A threshold of 20mm between skin and deltoid muscle allows 5mm penetration of a standard 25mm needle into the deltoid muscle. Our findings suggest that the proportion of adults with obesity that may require a longer-than-standard (>25mm) needle to achieve intramuscular delivery was 45%, 40% and 15% for the New Zealand, Australian and USA injection sites respectively. In the context of mass vaccination, this suggests that a substantive proportion of adults require longer-than-standard needles for vaccines that require intramuscular delivery, and that this proportion is greater for vaccination guidance recommending injection sites more distal from the acromion such as Australia and New Zealand. This finding highlights the importance of awareness among vaccine administrators and vaccine recipients to consider using or requesting a longer needle to ensure intramuscular vaccine delivery. This is relevant not only for the mRNA vaccines being administered in global efforts to alleviate the burden of the COVID-19 pandemic,[[22]] but also the numerous other vaccines recommended for intramuscular injection.[[11,15]]
Sex is also an important determinant of skin-to-deltoid-muscle distance and may help inform needle length choice. The mean skin-to-deltoid-muscle distance measurements were greater for females, whereas the differences in skin-to-deltoid-muscle distance between the three studied injection sites were larger for males. The current study found a positive association between skin-to-deltoid-muscle distance and each of arm circumference and BMI. The estimated increases in skin-to-deltoid-muscle distance per 1cm in arm circumference, and one unit in BMI were 0.77mm and 0.65mm respectively. For example, a 6.5cm increase in arm circumference and a 7.7 unit increase in BMI correspond to an extra 5mm skin-to-deltoid-muscle difference. Both measurements could be used in practice to predict an individual’s skin-to-deltoid-muscle distance to inform needle length choice, however, measuring one’s arm circumference may be preferred. Arm circumference is quick, non-invasive, easy to learn, does not require a calculation and may provoke less stigmatisation compared to using BMI to predict skin-to-deltoid-muscle distance. These findings support the derivation and use of different sex-based BMI and arm circumference thresholds for recommending standard or longer needle length for intramuscular injection. This study was not large enough to determine thresholds of arm circumference or BMI, with adequate sensitivity and specificity, that might also determine appropriate needle length.
This study shows that skin-to-deltoid-muscle distance increases at injection sites more distal from the acromion process, and closer to the deltoid tuberosity, which is consistent with findings from other clinical studies.[[5,19,20]] Injection sites further from the acromion process reduce the risk of the needle hitting the axillary nerve and posterior circumflex humeral artery.[[19]]
Limitations
Our results may not be generalisable to populations with different age ranges, ethnicities and comorbidities, characteristics that may influence body fat and muscle distributions.[[23]] Our study did not include children and disproportionately included those with respiratory disease comorbidities, which reflects our institution’s participant database. Studies with larger sample sizes in different populations are needed to enable predictive models for an individual’s skin-to-deltoid-muscle distance to be derived. This study had insufficient power to provide estimates of the arm circumference and BMI cut-points where a needle longer than the standard 25mm length is required to ensure intramuscular delivery of a vaccine in the deltoid muscle.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied in a population with obesity. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Summary
Abstract
Aim
Worldwide, immunisation guidelines variably locate the deltoid injection site based on anatomical landmarks. This may influence the skin-to-deltoid-muscle distance and therefore the needle length required to achieve intramuscular injection. Obesity is associated with increased skin-to-deltoid-muscle distance, but it is unknown whether the injection site location chosen in individuals with obesity impacts the needle length required for intramuscular injection. The aim of the study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by the national guidelines of the United States of America (USA), Australia and New Zealand, in obese adults. The study also explored i) the associations between skin-to-deltoid-muscle distance across the three recommended sites with sex, body mass index (BMI), and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20 millimetres (mm), in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Method
Non-interventional cross-sectionalstudy in a single site, non-clinical setting in Wellington, New Zealand. Forty participants (29 females), aged ≥18years, with obesity (BMI>30 kilograms [km]/m[[2]]). Measurements included distance from acromionto injection sites, BMI, arm circumference, and skin-to-deltoid-muscle distancemeasured by ultrasound at each recommended injection site.
Results
Mean (SD) skin-to-deltoid-muscle distances for USA, Australia and New Zealand sites were 13.96mm (4.54), 17.94mm (6.08) and 20.26mm (5.91) respectively, with a mean (95% confidence interval) for the distance between Australia minus New Zealand -2.7mm (-3.5 to -1.9), P<0.001; and USA minus New Zealand -7.6 mm (-8.5 to -6.7); P<0.001. Skin-to-deltoid-muscle distance was greater in females and was positively associated with BMI and arm circumference. The proportions with a skin-to-deltoid-muscle distance >20 mm were 45%, 40% and 15% for the New Zealand, Australia and USA sites respectively. However, the sample size was relatively small, limiting interpretation in specific sub-groups.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Author Information
Acknowledgements
Correspondence
Correspondence Email
Competing Interests
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Skin-to-deltoid-muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study
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Messenger ribonucleic acid (mRNA) vaccines developed to protect against SARS-CoV-2 infection are only approved for intramuscular administration.[[1,2]] A needle of sufficient length to penetrate through the skin and subcutaneous tissue is needed to reach the deltoid muscle (the skin-to-deltoid-muscle distance), which is the preferred injection site.[[3]] It has been suggested that penetration of the deltoid muscle by 5 millimetres (mm) is required to ensure intramuscular deposition of the vaccine.[[4]] With national rates of adults with obesity above 40% in the United States of America (USA),[[5]] and around a third in Australia[[6]] and New Zealand,[[7]] it is important to choose an appropriate needle length for this demographic. Published data report that a standard needle length (25mm) is suitable for most people with a body mass index (BMI) <25 kilograms [kg]/m2.[[4,8]] However, progressively higher BMIs increase the likelihood of requiring a longer-than-standard needle length for deltoid intramuscular injection.[[8–10]]
Worldwide, immunisation guidelines vary in their instructions on how to choose the correct needle length based on BMI and body weight, or contain non-specific terms such as “larger arms”.[[11–13]] An accurate measurement of BMI for a vaccine recipient is not always readily available and the interpretation of arm size is subjective, resulting in an increased risk of inappropriate needle length choice and subcutaneous vaccine delivery. An observational study of a SARS-CoV-2 mRNA vaccine, administered with needles of different lengths at the discretion of the vaccinator, did not demonstrate a difference in immunogenicity between those vaccinated with a needle of sufficient versus insufficient length to achieve intramuscular deposition of vaccine.[[14]] However, there is evidence that intramuscular injection results in significantly better immune response compared to subcutaneous delivery of influenza and hepatitis B vaccines.[[15]] Further, there is high-grade evidence that subcutaneous administration of different vaccine types (adjuvanted, live virus and non-adjuvanted) is associated with increased local side effects including abscess and granuloma formation, compared to intramuscular delivery.[[15–17]]
The location of the deltoid intramuscular injection site is defined variably between countries based on anatomical landmarks (Figure 1). In the USA, the recommended injection site is 2 inches (approximately 5 centimetres [cm]) below the acromion process and above the axillary fold.[[13]] In Australia, the site recommended for injection is the midpoint between the acromion process and the deltoid tuberosity.[[11]] In New Zealand, the site recommended is at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting the acromion and the deltoid tuberosity.[[18]]
Current guidance on needle length choice does not account for potential variation in skin-to-deltoid-muscle distance across the length of the deltoid muscle. In two studies,[[19,20]] a statistically significant difference in mean skin-to-deltoid-muscle distance between injection sites was reported in individuals with a low/normal BMI of approximately 20kg/m[[2]]. With the relatively small skin-to-deltoid-muscle distances in this population, the absolute differences were not clinically important for intramuscular injection needle length recommendations. Whether the same variability in skin-to-deltoid-muscle distance exists in a population with obesity is unknown, yet highly relevant since they are at risk of subcutaneous injection from insufficient needle length. If within-subject differences in skin-to-deltoid-muscle distance at larger BMIs are of the same relative magnitudes to that reported by Nakajima et al.,[[18,19]] the injection site chosen may alter the required needle length. It is therefore important to understand in adults with obesity the difference in skin-to-deltoid-muscle distance between recommended injection sites worldwide to help inform correct choice of needle length.
The objective of this study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by national guidelines in the USA, Australia and New Zealand. The study also aimed to explore if any identified differences in skin-to-deltoid-muscle distance between the three recommended injection sites differed by sex, BMI or arm circumference.
Subjects
Eligible participants were 18 years or older, of any ethnicity, with a BMI of 30kg/m[[2]] or more, who provided informed consent prior to participation. Recruitment took place by direct invitation of potential participants on the Medical Research Institute of New Zealand (MRINZ) database and by advertisement on social media. No stopping criteria applied, provided participants did not withdraw consent before completion of their study visit.
Materials and methods
This was a single-site non-interventional cross-sectional study conducted at the MRINZ in Wellington, New Zealand. All investigations were completed in a single visit of approximately 45 minutes after obtaining informed consent.
Participant date of birth was used to determine age. Participant self-reported ethnicity, side of non-dominant arm and comorbidities were recorded. Ethnicity is reported as prioritised ethnicity using Level 1 codes, a method of reducing multiple ethnicities for analysis in the health and disability sector, which is used in the Statistics New Zealand Census.[[21]] Participant height and weight were measured by using a calibrated stadiometer and body scale (BWB-800, Wedderburn, New Zealand). Derived BMI was calculated.
Participants were instructed to expose their non-dominant arm and hang it relaxed by their side. First, the acromion process was identified by palpation and a mark was placed with an indelible pen (point A), followed by a mark on the skin corresponding with the deltoid tuberosity (point D). The USA injection site (point USA) was identified and marked by measuring 5cm inferior to point A, followed by the Australian injection site (point AUS) at the exact midpoint between the point A and point D, and the New Zealand injection site (point NZ) at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting point A and point D (Figure 1). All markings were checked by a second investigator before the distances were measured (in cm) between all points.
For each participant, with the shoulder in anatomical position and elbow passively flexed and slightly pronated, three ultrasound images displaying the skin, subcutaneous tissue and fascia and the deltoid muscle were captured and saved using a high-frequency (13-6 MHz) linear transducer (Sonosite X-Porte, Fujifilm, Japan), after using sufficient water-soluble ultrasound transmission gel as an acoustic standoff. The middle of the ultrasound probe was placed at the marked injection site, with minimal pressure, at a 90-degree angle with the skin, in the coronal plane. Penetration depth setting was increased as required to ensure a sufficient volume of the deltoid muscle was displayed. Ultrasound images were obtained by trained clinical staff (LK, SK). Measurements of the distance (in mm, to the nearest whole mm) between the skin and the fascia of the deltoid muscle were performed by a radiology registrar (JB). For each participant, two consecutive measurements of arm circumference were performed at point USA, point AUS and point NZ with their shoulder in 90 degrees passive abduction.
The primary outcome was the difference in skin-to-deltoid-muscle-distance (in millimetres) between three different sites recommended for intramuscular vaccination. The secondary outcomes included i) associations between skin-to-deltoid-muscle distance across the three recommended injection sites, with sex, BMI and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20mm, in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Sample size
The sample size was based on publications by Nakajima et al.,[[19,20]] with a view to detect a difference of 0.6mm with 80% power and a two-sided type I error rate of 5%, and further increased to allow for four explanatory degrees of freedom (distance from acromion, sex and arm circumference). An analysis was undertaken after recruitment of 40 participants to assess the size of the paired SD, which was sufficient to end recruitment at this point.
Data analysis
Continuous data were described by mean and standard deviation (SD), median and inter-quartile range (IQR) and minimum (min) to maximum (max). Proportions were described by counts and proportions expressed as percentages. On the scatter plot, linear regression lines are shown. The estimates of the associations between skin-to-deltoid muscle distance are by mixed linear models with fixed effects for measurement site, sex and arm circumference or BMI; together with associated interaction terms and, as random effects, the individual participants with unstructured variance-covariance correlation structures for the repeated measurements. The difference in paired proportions was estimated by appropriate categorical data models for the proportion of participants with skin-to-deltoid-muscle distances above nominated thresholds.
A threshold of p<0.05 was used to determine if between-site differences in mean skin-to-deltoid-muscle distance were statistically significant. The primary comparisons used the New Zealand definition of injection site as the reference level and compared this to the Australia and USA sites (Hypothesis 1). The secondary analyses included a one degree of freedom test using the ordinal rank of the distance from the acromion process as a predictor to explore if skin-to-deltoid-muscle distance increases as the distance increases (Hypothesis 2). To explore the possibility that the difference in skin-to-deltoid-muscle distance between sites differed by sex (Hypothesis 3), BMI (Hypothesis 4), and arm circumference (Hypothesis 5) were explored using mixed linear models with main effects and interaction terms for each of these possible effect modifying predictors. A comparison of the paired proportions was done by a generalised mixed linear model (Hypothesis 6). SAS version 9.4 was used for statistical analyses.
Ethical approval
This study was approved by the Northern B Health and Disability Ethics Committee (REF: 2022 EXP 12121). All participants provided written informed consent prior to participation.
Results
View Figures 1–2 and Tables 1–3.
There were 29 female and 11 male (n=40) participants with a mean (SD) age of 52.0 (16.3) years. There were no missing data. Demographic and anthropometric data are shown in Table 1. Mean (SD) BMI was 36.8 (5.0) kg/m[[2]] and the mean (SD) skin-to-deltoid-muscle distance for the USA, Australia and New Zealand sites were 13.96 (4.54), 17.94 (6.08) and 20.26 (5.91) mm respectively. Note that a recommended needle length is one that is 5mm greater than the skin-to-deltoid-muscle distance. Using prioritised ethnicity, 90% of participants were European, 5% were Māori, 2.5% were Pasifika, and 2.5% were of Middle Eastern/Latin American/African ethnicity. Respiratory disease was the most common comorbidity, being present in 70%, followed by cardiovascular disease in 27.5% and diabetes in 5% of the study population. This table also shows variability in the distances to injection sites in relation to anatomical landmarks recommended by the Australian and New Zealand guidelines and the summary data of the arm circumference measured at each injection site.
Differences in skin-to-deltoid muscle distance between recommended injection sites
The estimates of the differences between skin-to-deltoid-muscle distance ultrasound measurements between injection sites are displayed in Table 2. There was an interaction between site of measurement of skin-to-deltoid-muscle distance and sex, P-interaction 0.035; the sex-specific differences are also shown. The interpretation is that skin-to-deltoid-muscle distance is larger for the New Zealand than the Australia and USA sites and that these differences between injections sites are in turn larger for men compared to women.
Skin-to-deltoid-muscle distance in relation to anthropometric measurements
Distance from acromion
When the three measurement sites are treated as ordinal scale variables one unit apart in the order of increasing distance from the acromion (USA then Australia then New Zealand sites), the estimate of increase in skin-to-deltoid-muscle distance per “unit” increase in measurement site is 3.1mm (95% CI 2.7 to 3.6).
Arm circumference
Increasing arm circumference was associated with greater skin-to-deltoid-muscle distance for all three injection sites (Figure 2a and Figure 2b) independent of sex (p=0.56), measurement site (p=0.30), or the combination of sex and measurement site (p=0.39). The estimated coefficient (95% CI) was 0.77 (0.51 to 0.97) mm increase in skin-to-deltoid-muscle distance per 1cm increase in arm circumference.
BMI
There was a positive association between skin-to-deltoid-muscle distance and BMI for all three injection sites (Figure 3c and Figure 3d), independent of sex (p=0.33), or measurement site (p=0.27) or the combination of sex and measurement site (p=0.10). The estimated slope (95% CI) was 0.65 (0.40 to 0.90) mm increase in skin-to-deltoid-muscle distance per unit increase in BMI.
Proportions of participants with skin-to-deltoid-muscle distance greater than 20mm and 33mm
The two standard needle lengths in New Zealand are 25mm and 38mm. After allowing for 5mm penetration into the deltoid muscle these needle lengths correspond to skin-to-deltoid-muscle distances of 20mm and 33mm. No participant had a skin-to-deltoid-muscle distance of greater than 33mm at any measurement site, and so based on this sample no participant would require a needle length greater than 38mm. The counts and proportions of participants with a skin-to-deltoid-muscle distance greater than 20mm, and therefore needing a needle length of greater than 25mm, including cross-classification by measurement site, are shown in Table 3. These proportions were 45% for the New Zealand, 40% for the Australian and 15% for the USA measurement sites respectively. The differences in paired proportions (95% CI) were USA versus New Zealand, 30% (15.8 to 44.2), p<0.001, and Australia versus New Zealand, 5% (-1.8 to 11.8), p=0.15.
Discussion
This study has identified that in adults with obesity, defined as a BMI above 30kg/m[[2]], there are marked differences in the skin-to-deltoid-muscle distance measured at three recommended sites for deltoid intramuscular vaccine injection. The magnitude of the differences was of clinical importance, with the New Zealand site having a mean skin-to-deltoid-muscle distance approximately 6mm and 4mm greater than the recommended sites in USA and Australian guidance respectively. Skin-to-deltoid-muscle distance was greater in females, and participants with greater BMI and larger arm circumference were likely to have greater skin-to-deltoid-muscle distances. These findings suggest that when choosing the appropriate needle length to achieve intramuscular injection in a population with obesity, consideration needs to be given to the specific deltoid injection site selected, as well as the sex, BMI and/or the arm circumference of the individual.
The mean skin-to-deltoid-muscle distance (SD) was 14.0mm (4.5), 17.9mm (6.1) and 20.3mm (5.9) at the recommended USA, Australian and New Zealand injection site respectively. These measures can be considered in relation to the standard 25mm needle used for intramuscular injection of vaccine in clinical practice. A threshold of 20mm between skin and deltoid muscle allows 5mm penetration of a standard 25mm needle into the deltoid muscle. Our findings suggest that the proportion of adults with obesity that may require a longer-than-standard (>25mm) needle to achieve intramuscular delivery was 45%, 40% and 15% for the New Zealand, Australian and USA injection sites respectively. In the context of mass vaccination, this suggests that a substantive proportion of adults require longer-than-standard needles for vaccines that require intramuscular delivery, and that this proportion is greater for vaccination guidance recommending injection sites more distal from the acromion such as Australia and New Zealand. This finding highlights the importance of awareness among vaccine administrators and vaccine recipients to consider using or requesting a longer needle to ensure intramuscular vaccine delivery. This is relevant not only for the mRNA vaccines being administered in global efforts to alleviate the burden of the COVID-19 pandemic,[[22]] but also the numerous other vaccines recommended for intramuscular injection.[[11,15]]
Sex is also an important determinant of skin-to-deltoid-muscle distance and may help inform needle length choice. The mean skin-to-deltoid-muscle distance measurements were greater for females, whereas the differences in skin-to-deltoid-muscle distance between the three studied injection sites were larger for males. The current study found a positive association between skin-to-deltoid-muscle distance and each of arm circumference and BMI. The estimated increases in skin-to-deltoid-muscle distance per 1cm in arm circumference, and one unit in BMI were 0.77mm and 0.65mm respectively. For example, a 6.5cm increase in arm circumference and a 7.7 unit increase in BMI correspond to an extra 5mm skin-to-deltoid-muscle difference. Both measurements could be used in practice to predict an individual’s skin-to-deltoid-muscle distance to inform needle length choice, however, measuring one’s arm circumference may be preferred. Arm circumference is quick, non-invasive, easy to learn, does not require a calculation and may provoke less stigmatisation compared to using BMI to predict skin-to-deltoid-muscle distance. These findings support the derivation and use of different sex-based BMI and arm circumference thresholds for recommending standard or longer needle length for intramuscular injection. This study was not large enough to determine thresholds of arm circumference or BMI, with adequate sensitivity and specificity, that might also determine appropriate needle length.
This study shows that skin-to-deltoid-muscle distance increases at injection sites more distal from the acromion process, and closer to the deltoid tuberosity, which is consistent with findings from other clinical studies.[[5,19,20]] Injection sites further from the acromion process reduce the risk of the needle hitting the axillary nerve and posterior circumflex humeral artery.[[19]]
Limitations
Our results may not be generalisable to populations with different age ranges, ethnicities and comorbidities, characteristics that may influence body fat and muscle distributions.[[23]] Our study did not include children and disproportionately included those with respiratory disease comorbidities, which reflects our institution’s participant database. Studies with larger sample sizes in different populations are needed to enable predictive models for an individual’s skin-to-deltoid-muscle distance to be derived. This study had insufficient power to provide estimates of the arm circumference and BMI cut-points where a needle longer than the standard 25mm length is required to ensure intramuscular delivery of a vaccine in the deltoid muscle.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied in a population with obesity. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Summary
Abstract
Aim
Worldwide, immunisation guidelines variably locate the deltoid injection site based on anatomical landmarks. This may influence the skin-to-deltoid-muscle distance and therefore the needle length required to achieve intramuscular injection. Obesity is associated with increased skin-to-deltoid-muscle distance, but it is unknown whether the injection site location chosen in individuals with obesity impacts the needle length required for intramuscular injection. The aim of the study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by the national guidelines of the United States of America (USA), Australia and New Zealand, in obese adults. The study also explored i) the associations between skin-to-deltoid-muscle distance across the three recommended sites with sex, body mass index (BMI), and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20 millimetres (mm), in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Method
Non-interventional cross-sectionalstudy in a single site, non-clinical setting in Wellington, New Zealand. Forty participants (29 females), aged ≥18years, with obesity (BMI>30 kilograms [km]/m[[2]]). Measurements included distance from acromionto injection sites, BMI, arm circumference, and skin-to-deltoid-muscle distancemeasured by ultrasound at each recommended injection site.
Results
Mean (SD) skin-to-deltoid-muscle distances for USA, Australia and New Zealand sites were 13.96mm (4.54), 17.94mm (6.08) and 20.26mm (5.91) respectively, with a mean (95% confidence interval) for the distance between Australia minus New Zealand -2.7mm (-3.5 to -1.9), P<0.001; and USA minus New Zealand -7.6 mm (-8.5 to -6.7); P<0.001. Skin-to-deltoid-muscle distance was greater in females and was positively associated with BMI and arm circumference. The proportions with a skin-to-deltoid-muscle distance >20 mm were 45%, 40% and 15% for the New Zealand, Australia and USA sites respectively. However, the sample size was relatively small, limiting interpretation in specific sub-groups.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Author Information
Acknowledgements
Correspondence
Correspondence Email
Competing Interests
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Skin-to-deltoid-muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study
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Messenger ribonucleic acid (mRNA) vaccines developed to protect against SARS-CoV-2 infection are only approved for intramuscular administration.[[1,2]] A needle of sufficient length to penetrate through the skin and subcutaneous tissue is needed to reach the deltoid muscle (the skin-to-deltoid-muscle distance), which is the preferred injection site.[[3]] It has been suggested that penetration of the deltoid muscle by 5 millimetres (mm) is required to ensure intramuscular deposition of the vaccine.[[4]] With national rates of adults with obesity above 40% in the United States of America (USA),[[5]] and around a third in Australia[[6]] and New Zealand,[[7]] it is important to choose an appropriate needle length for this demographic. Published data report that a standard needle length (25mm) is suitable for most people with a body mass index (BMI) <25 kilograms [kg]/m2.[[4,8]] However, progressively higher BMIs increase the likelihood of requiring a longer-than-standard needle length for deltoid intramuscular injection.[[8–10]]
Worldwide, immunisation guidelines vary in their instructions on how to choose the correct needle length based on BMI and body weight, or contain non-specific terms such as “larger arms”.[[11–13]] An accurate measurement of BMI for a vaccine recipient is not always readily available and the interpretation of arm size is subjective, resulting in an increased risk of inappropriate needle length choice and subcutaneous vaccine delivery. An observational study of a SARS-CoV-2 mRNA vaccine, administered with needles of different lengths at the discretion of the vaccinator, did not demonstrate a difference in immunogenicity between those vaccinated with a needle of sufficient versus insufficient length to achieve intramuscular deposition of vaccine.[[14]] However, there is evidence that intramuscular injection results in significantly better immune response compared to subcutaneous delivery of influenza and hepatitis B vaccines.[[15]] Further, there is high-grade evidence that subcutaneous administration of different vaccine types (adjuvanted, live virus and non-adjuvanted) is associated with increased local side effects including abscess and granuloma formation, compared to intramuscular delivery.[[15–17]]
The location of the deltoid intramuscular injection site is defined variably between countries based on anatomical landmarks (Figure 1). In the USA, the recommended injection site is 2 inches (approximately 5 centimetres [cm]) below the acromion process and above the axillary fold.[[13]] In Australia, the site recommended for injection is the midpoint between the acromion process and the deltoid tuberosity.[[11]] In New Zealand, the site recommended is at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting the acromion and the deltoid tuberosity.[[18]]
Current guidance on needle length choice does not account for potential variation in skin-to-deltoid-muscle distance across the length of the deltoid muscle. In two studies,[[19,20]] a statistically significant difference in mean skin-to-deltoid-muscle distance between injection sites was reported in individuals with a low/normal BMI of approximately 20kg/m[[2]]. With the relatively small skin-to-deltoid-muscle distances in this population, the absolute differences were not clinically important for intramuscular injection needle length recommendations. Whether the same variability in skin-to-deltoid-muscle distance exists in a population with obesity is unknown, yet highly relevant since they are at risk of subcutaneous injection from insufficient needle length. If within-subject differences in skin-to-deltoid-muscle distance at larger BMIs are of the same relative magnitudes to that reported by Nakajima et al.,[[18,19]] the injection site chosen may alter the required needle length. It is therefore important to understand in adults with obesity the difference in skin-to-deltoid-muscle distance between recommended injection sites worldwide to help inform correct choice of needle length.
The objective of this study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by national guidelines in the USA, Australia and New Zealand. The study also aimed to explore if any identified differences in skin-to-deltoid-muscle distance between the three recommended injection sites differed by sex, BMI or arm circumference.
Subjects
Eligible participants were 18 years or older, of any ethnicity, with a BMI of 30kg/m[[2]] or more, who provided informed consent prior to participation. Recruitment took place by direct invitation of potential participants on the Medical Research Institute of New Zealand (MRINZ) database and by advertisement on social media. No stopping criteria applied, provided participants did not withdraw consent before completion of their study visit.
Materials and methods
This was a single-site non-interventional cross-sectional study conducted at the MRINZ in Wellington, New Zealand. All investigations were completed in a single visit of approximately 45 minutes after obtaining informed consent.
Participant date of birth was used to determine age. Participant self-reported ethnicity, side of non-dominant arm and comorbidities were recorded. Ethnicity is reported as prioritised ethnicity using Level 1 codes, a method of reducing multiple ethnicities for analysis in the health and disability sector, which is used in the Statistics New Zealand Census.[[21]] Participant height and weight were measured by using a calibrated stadiometer and body scale (BWB-800, Wedderburn, New Zealand). Derived BMI was calculated.
Participants were instructed to expose their non-dominant arm and hang it relaxed by their side. First, the acromion process was identified by palpation and a mark was placed with an indelible pen (point A), followed by a mark on the skin corresponding with the deltoid tuberosity (point D). The USA injection site (point USA) was identified and marked by measuring 5cm inferior to point A, followed by the Australian injection site (point AUS) at the exact midpoint between the point A and point D, and the New Zealand injection site (point NZ) at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting point A and point D (Figure 1). All markings were checked by a second investigator before the distances were measured (in cm) between all points.
For each participant, with the shoulder in anatomical position and elbow passively flexed and slightly pronated, three ultrasound images displaying the skin, subcutaneous tissue and fascia and the deltoid muscle were captured and saved using a high-frequency (13-6 MHz) linear transducer (Sonosite X-Porte, Fujifilm, Japan), after using sufficient water-soluble ultrasound transmission gel as an acoustic standoff. The middle of the ultrasound probe was placed at the marked injection site, with minimal pressure, at a 90-degree angle with the skin, in the coronal plane. Penetration depth setting was increased as required to ensure a sufficient volume of the deltoid muscle was displayed. Ultrasound images were obtained by trained clinical staff (LK, SK). Measurements of the distance (in mm, to the nearest whole mm) between the skin and the fascia of the deltoid muscle were performed by a radiology registrar (JB). For each participant, two consecutive measurements of arm circumference were performed at point USA, point AUS and point NZ with their shoulder in 90 degrees passive abduction.
The primary outcome was the difference in skin-to-deltoid-muscle-distance (in millimetres) between three different sites recommended for intramuscular vaccination. The secondary outcomes included i) associations between skin-to-deltoid-muscle distance across the three recommended injection sites, with sex, BMI and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20mm, in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Sample size
The sample size was based on publications by Nakajima et al.,[[19,20]] with a view to detect a difference of 0.6mm with 80% power and a two-sided type I error rate of 5%, and further increased to allow for four explanatory degrees of freedom (distance from acromion, sex and arm circumference). An analysis was undertaken after recruitment of 40 participants to assess the size of the paired SD, which was sufficient to end recruitment at this point.
Data analysis
Continuous data were described by mean and standard deviation (SD), median and inter-quartile range (IQR) and minimum (min) to maximum (max). Proportions were described by counts and proportions expressed as percentages. On the scatter plot, linear regression lines are shown. The estimates of the associations between skin-to-deltoid muscle distance are by mixed linear models with fixed effects for measurement site, sex and arm circumference or BMI; together with associated interaction terms and, as random effects, the individual participants with unstructured variance-covariance correlation structures for the repeated measurements. The difference in paired proportions was estimated by appropriate categorical data models for the proportion of participants with skin-to-deltoid-muscle distances above nominated thresholds.
A threshold of p<0.05 was used to determine if between-site differences in mean skin-to-deltoid-muscle distance were statistically significant. The primary comparisons used the New Zealand definition of injection site as the reference level and compared this to the Australia and USA sites (Hypothesis 1). The secondary analyses included a one degree of freedom test using the ordinal rank of the distance from the acromion process as a predictor to explore if skin-to-deltoid-muscle distance increases as the distance increases (Hypothesis 2). To explore the possibility that the difference in skin-to-deltoid-muscle distance between sites differed by sex (Hypothesis 3), BMI (Hypothesis 4), and arm circumference (Hypothesis 5) were explored using mixed linear models with main effects and interaction terms for each of these possible effect modifying predictors. A comparison of the paired proportions was done by a generalised mixed linear model (Hypothesis 6). SAS version 9.4 was used for statistical analyses.
Ethical approval
This study was approved by the Northern B Health and Disability Ethics Committee (REF: 2022 EXP 12121). All participants provided written informed consent prior to participation.
Results
View Figures 1–2 and Tables 1–3.
There were 29 female and 11 male (n=40) participants with a mean (SD) age of 52.0 (16.3) years. There were no missing data. Demographic and anthropometric data are shown in Table 1. Mean (SD) BMI was 36.8 (5.0) kg/m[[2]] and the mean (SD) skin-to-deltoid-muscle distance for the USA, Australia and New Zealand sites were 13.96 (4.54), 17.94 (6.08) and 20.26 (5.91) mm respectively. Note that a recommended needle length is one that is 5mm greater than the skin-to-deltoid-muscle distance. Using prioritised ethnicity, 90% of participants were European, 5% were Māori, 2.5% were Pasifika, and 2.5% were of Middle Eastern/Latin American/African ethnicity. Respiratory disease was the most common comorbidity, being present in 70%, followed by cardiovascular disease in 27.5% and diabetes in 5% of the study population. This table also shows variability in the distances to injection sites in relation to anatomical landmarks recommended by the Australian and New Zealand guidelines and the summary data of the arm circumference measured at each injection site.
Differences in skin-to-deltoid muscle distance between recommended injection sites
The estimates of the differences between skin-to-deltoid-muscle distance ultrasound measurements between injection sites are displayed in Table 2. There was an interaction between site of measurement of skin-to-deltoid-muscle distance and sex, P-interaction 0.035; the sex-specific differences are also shown. The interpretation is that skin-to-deltoid-muscle distance is larger for the New Zealand than the Australia and USA sites and that these differences between injections sites are in turn larger for men compared to women.
Skin-to-deltoid-muscle distance in relation to anthropometric measurements
Distance from acromion
When the three measurement sites are treated as ordinal scale variables one unit apart in the order of increasing distance from the acromion (USA then Australia then New Zealand sites), the estimate of increase in skin-to-deltoid-muscle distance per “unit” increase in measurement site is 3.1mm (95% CI 2.7 to 3.6).
Arm circumference
Increasing arm circumference was associated with greater skin-to-deltoid-muscle distance for all three injection sites (Figure 2a and Figure 2b) independent of sex (p=0.56), measurement site (p=0.30), or the combination of sex and measurement site (p=0.39). The estimated coefficient (95% CI) was 0.77 (0.51 to 0.97) mm increase in skin-to-deltoid-muscle distance per 1cm increase in arm circumference.
BMI
There was a positive association between skin-to-deltoid-muscle distance and BMI for all three injection sites (Figure 3c and Figure 3d), independent of sex (p=0.33), or measurement site (p=0.27) or the combination of sex and measurement site (p=0.10). The estimated slope (95% CI) was 0.65 (0.40 to 0.90) mm increase in skin-to-deltoid-muscle distance per unit increase in BMI.
Proportions of participants with skin-to-deltoid-muscle distance greater than 20mm and 33mm
The two standard needle lengths in New Zealand are 25mm and 38mm. After allowing for 5mm penetration into the deltoid muscle these needle lengths correspond to skin-to-deltoid-muscle distances of 20mm and 33mm. No participant had a skin-to-deltoid-muscle distance of greater than 33mm at any measurement site, and so based on this sample no participant would require a needle length greater than 38mm. The counts and proportions of participants with a skin-to-deltoid-muscle distance greater than 20mm, and therefore needing a needle length of greater than 25mm, including cross-classification by measurement site, are shown in Table 3. These proportions were 45% for the New Zealand, 40% for the Australian and 15% for the USA measurement sites respectively. The differences in paired proportions (95% CI) were USA versus New Zealand, 30% (15.8 to 44.2), p<0.001, and Australia versus New Zealand, 5% (-1.8 to 11.8), p=0.15.
Discussion
This study has identified that in adults with obesity, defined as a BMI above 30kg/m[[2]], there are marked differences in the skin-to-deltoid-muscle distance measured at three recommended sites for deltoid intramuscular vaccine injection. The magnitude of the differences was of clinical importance, with the New Zealand site having a mean skin-to-deltoid-muscle distance approximately 6mm and 4mm greater than the recommended sites in USA and Australian guidance respectively. Skin-to-deltoid-muscle distance was greater in females, and participants with greater BMI and larger arm circumference were likely to have greater skin-to-deltoid-muscle distances. These findings suggest that when choosing the appropriate needle length to achieve intramuscular injection in a population with obesity, consideration needs to be given to the specific deltoid injection site selected, as well as the sex, BMI and/or the arm circumference of the individual.
The mean skin-to-deltoid-muscle distance (SD) was 14.0mm (4.5), 17.9mm (6.1) and 20.3mm (5.9) at the recommended USA, Australian and New Zealand injection site respectively. These measures can be considered in relation to the standard 25mm needle used for intramuscular injection of vaccine in clinical practice. A threshold of 20mm between skin and deltoid muscle allows 5mm penetration of a standard 25mm needle into the deltoid muscle. Our findings suggest that the proportion of adults with obesity that may require a longer-than-standard (>25mm) needle to achieve intramuscular delivery was 45%, 40% and 15% for the New Zealand, Australian and USA injection sites respectively. In the context of mass vaccination, this suggests that a substantive proportion of adults require longer-than-standard needles for vaccines that require intramuscular delivery, and that this proportion is greater for vaccination guidance recommending injection sites more distal from the acromion such as Australia and New Zealand. This finding highlights the importance of awareness among vaccine administrators and vaccine recipients to consider using or requesting a longer needle to ensure intramuscular vaccine delivery. This is relevant not only for the mRNA vaccines being administered in global efforts to alleviate the burden of the COVID-19 pandemic,[[22]] but also the numerous other vaccines recommended for intramuscular injection.[[11,15]]
Sex is also an important determinant of skin-to-deltoid-muscle distance and may help inform needle length choice. The mean skin-to-deltoid-muscle distance measurements were greater for females, whereas the differences in skin-to-deltoid-muscle distance between the three studied injection sites were larger for males. The current study found a positive association between skin-to-deltoid-muscle distance and each of arm circumference and BMI. The estimated increases in skin-to-deltoid-muscle distance per 1cm in arm circumference, and one unit in BMI were 0.77mm and 0.65mm respectively. For example, a 6.5cm increase in arm circumference and a 7.7 unit increase in BMI correspond to an extra 5mm skin-to-deltoid-muscle difference. Both measurements could be used in practice to predict an individual’s skin-to-deltoid-muscle distance to inform needle length choice, however, measuring one’s arm circumference may be preferred. Arm circumference is quick, non-invasive, easy to learn, does not require a calculation and may provoke less stigmatisation compared to using BMI to predict skin-to-deltoid-muscle distance. These findings support the derivation and use of different sex-based BMI and arm circumference thresholds for recommending standard or longer needle length for intramuscular injection. This study was not large enough to determine thresholds of arm circumference or BMI, with adequate sensitivity and specificity, that might also determine appropriate needle length.
This study shows that skin-to-deltoid-muscle distance increases at injection sites more distal from the acromion process, and closer to the deltoid tuberosity, which is consistent with findings from other clinical studies.[[5,19,20]] Injection sites further from the acromion process reduce the risk of the needle hitting the axillary nerve and posterior circumflex humeral artery.[[19]]
Limitations
Our results may not be generalisable to populations with different age ranges, ethnicities and comorbidities, characteristics that may influence body fat and muscle distributions.[[23]] Our study did not include children and disproportionately included those with respiratory disease comorbidities, which reflects our institution’s participant database. Studies with larger sample sizes in different populations are needed to enable predictive models for an individual’s skin-to-deltoid-muscle distance to be derived. This study had insufficient power to provide estimates of the arm circumference and BMI cut-points where a needle longer than the standard 25mm length is required to ensure intramuscular delivery of a vaccine in the deltoid muscle.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied in a population with obesity. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Summary
Abstract
Aim
Worldwide, immunisation guidelines variably locate the deltoid injection site based on anatomical landmarks. This may influence the skin-to-deltoid-muscle distance and therefore the needle length required to achieve intramuscular injection. Obesity is associated with increased skin-to-deltoid-muscle distance, but it is unknown whether the injection site location chosen in individuals with obesity impacts the needle length required for intramuscular injection. The aim of the study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by the national guidelines of the United States of America (USA), Australia and New Zealand, in obese adults. The study also explored i) the associations between skin-to-deltoid-muscle distance across the three recommended sites with sex, body mass index (BMI), and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20 millimetres (mm), in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Method
Non-interventional cross-sectionalstudy in a single site, non-clinical setting in Wellington, New Zealand. Forty participants (29 females), aged ≥18years, with obesity (BMI>30 kilograms [km]/m[[2]]). Measurements included distance from acromionto injection sites, BMI, arm circumference, and skin-to-deltoid-muscle distancemeasured by ultrasound at each recommended injection site.
Results
Mean (SD) skin-to-deltoid-muscle distances for USA, Australia and New Zealand sites were 13.96mm (4.54), 17.94mm (6.08) and 20.26mm (5.91) respectively, with a mean (95% confidence interval) for the distance between Australia minus New Zealand -2.7mm (-3.5 to -1.9), P<0.001; and USA minus New Zealand -7.6 mm (-8.5 to -6.7); P<0.001. Skin-to-deltoid-muscle distance was greater in females and was positively associated with BMI and arm circumference. The proportions with a skin-to-deltoid-muscle distance >20 mm were 45%, 40% and 15% for the New Zealand, Australia and USA sites respectively. However, the sample size was relatively small, limiting interpretation in specific sub-groups.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Author Information
Acknowledgements
Correspondence
Correspondence Email
Competing Interests
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Contact diana@nzma.org.nz
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Skin-to-deltoid-muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study
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Messenger ribonucleic acid (mRNA) vaccines developed to protect against SARS-CoV-2 infection are only approved for intramuscular administration.[[1,2]] A needle of sufficient length to penetrate through the skin and subcutaneous tissue is needed to reach the deltoid muscle (the skin-to-deltoid-muscle distance), which is the preferred injection site.[[3]] It has been suggested that penetration of the deltoid muscle by 5 millimetres (mm) is required to ensure intramuscular deposition of the vaccine.[[4]] With national rates of adults with obesity above 40% in the United States of America (USA),[[5]] and around a third in Australia[[6]] and New Zealand,[[7]] it is important to choose an appropriate needle length for this demographic. Published data report that a standard needle length (25mm) is suitable for most people with a body mass index (BMI) <25 kilograms [kg]/m2.[[4,8]] However, progressively higher BMIs increase the likelihood of requiring a longer-than-standard needle length for deltoid intramuscular injection.[[8–10]]
Worldwide, immunisation guidelines vary in their instructions on how to choose the correct needle length based on BMI and body weight, or contain non-specific terms such as “larger arms”.[[11–13]] An accurate measurement of BMI for a vaccine recipient is not always readily available and the interpretation of arm size is subjective, resulting in an increased risk of inappropriate needle length choice and subcutaneous vaccine delivery. An observational study of a SARS-CoV-2 mRNA vaccine, administered with needles of different lengths at the discretion of the vaccinator, did not demonstrate a difference in immunogenicity between those vaccinated with a needle of sufficient versus insufficient length to achieve intramuscular deposition of vaccine.[[14]] However, there is evidence that intramuscular injection results in significantly better immune response compared to subcutaneous delivery of influenza and hepatitis B vaccines.[[15]] Further, there is high-grade evidence that subcutaneous administration of different vaccine types (adjuvanted, live virus and non-adjuvanted) is associated with increased local side effects including abscess and granuloma formation, compared to intramuscular delivery.[[15–17]]
The location of the deltoid intramuscular injection site is defined variably between countries based on anatomical landmarks (Figure 1). In the USA, the recommended injection site is 2 inches (approximately 5 centimetres [cm]) below the acromion process and above the axillary fold.[[13]] In Australia, the site recommended for injection is the midpoint between the acromion process and the deltoid tuberosity.[[11]] In New Zealand, the site recommended is at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting the acromion and the deltoid tuberosity.[[18]]
Current guidance on needle length choice does not account for potential variation in skin-to-deltoid-muscle distance across the length of the deltoid muscle. In two studies,[[19,20]] a statistically significant difference in mean skin-to-deltoid-muscle distance between injection sites was reported in individuals with a low/normal BMI of approximately 20kg/m[[2]]. With the relatively small skin-to-deltoid-muscle distances in this population, the absolute differences were not clinically important for intramuscular injection needle length recommendations. Whether the same variability in skin-to-deltoid-muscle distance exists in a population with obesity is unknown, yet highly relevant since they are at risk of subcutaneous injection from insufficient needle length. If within-subject differences in skin-to-deltoid-muscle distance at larger BMIs are of the same relative magnitudes to that reported by Nakajima et al.,[[18,19]] the injection site chosen may alter the required needle length. It is therefore important to understand in adults with obesity the difference in skin-to-deltoid-muscle distance between recommended injection sites worldwide to help inform correct choice of needle length.
The objective of this study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by national guidelines in the USA, Australia and New Zealand. The study also aimed to explore if any identified differences in skin-to-deltoid-muscle distance between the three recommended injection sites differed by sex, BMI or arm circumference.
Subjects
Eligible participants were 18 years or older, of any ethnicity, with a BMI of 30kg/m[[2]] or more, who provided informed consent prior to participation. Recruitment took place by direct invitation of potential participants on the Medical Research Institute of New Zealand (MRINZ) database and by advertisement on social media. No stopping criteria applied, provided participants did not withdraw consent before completion of their study visit.
Materials and methods
This was a single-site non-interventional cross-sectional study conducted at the MRINZ in Wellington, New Zealand. All investigations were completed in a single visit of approximately 45 minutes after obtaining informed consent.
Participant date of birth was used to determine age. Participant self-reported ethnicity, side of non-dominant arm and comorbidities were recorded. Ethnicity is reported as prioritised ethnicity using Level 1 codes, a method of reducing multiple ethnicities for analysis in the health and disability sector, which is used in the Statistics New Zealand Census.[[21]] Participant height and weight were measured by using a calibrated stadiometer and body scale (BWB-800, Wedderburn, New Zealand). Derived BMI was calculated.
Participants were instructed to expose their non-dominant arm and hang it relaxed by their side. First, the acromion process was identified by palpation and a mark was placed with an indelible pen (point A), followed by a mark on the skin corresponding with the deltoid tuberosity (point D). The USA injection site (point USA) was identified and marked by measuring 5cm inferior to point A, followed by the Australian injection site (point AUS) at the exact midpoint between the point A and point D, and the New Zealand injection site (point NZ) at the intersection of the axilla line (an imaginary line connecting the most superior point of the anterior and posterior axillary fold), and an imaginary line connecting point A and point D (Figure 1). All markings were checked by a second investigator before the distances were measured (in cm) between all points.
For each participant, with the shoulder in anatomical position and elbow passively flexed and slightly pronated, three ultrasound images displaying the skin, subcutaneous tissue and fascia and the deltoid muscle were captured and saved using a high-frequency (13-6 MHz) linear transducer (Sonosite X-Porte, Fujifilm, Japan), after using sufficient water-soluble ultrasound transmission gel as an acoustic standoff. The middle of the ultrasound probe was placed at the marked injection site, with minimal pressure, at a 90-degree angle with the skin, in the coronal plane. Penetration depth setting was increased as required to ensure a sufficient volume of the deltoid muscle was displayed. Ultrasound images were obtained by trained clinical staff (LK, SK). Measurements of the distance (in mm, to the nearest whole mm) between the skin and the fascia of the deltoid muscle were performed by a radiology registrar (JB). For each participant, two consecutive measurements of arm circumference were performed at point USA, point AUS and point NZ with their shoulder in 90 degrees passive abduction.
The primary outcome was the difference in skin-to-deltoid-muscle-distance (in millimetres) between three different sites recommended for intramuscular vaccination. The secondary outcomes included i) associations between skin-to-deltoid-muscle distance across the three recommended injection sites, with sex, BMI and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20mm, in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Sample size
The sample size was based on publications by Nakajima et al.,[[19,20]] with a view to detect a difference of 0.6mm with 80% power and a two-sided type I error rate of 5%, and further increased to allow for four explanatory degrees of freedom (distance from acromion, sex and arm circumference). An analysis was undertaken after recruitment of 40 participants to assess the size of the paired SD, which was sufficient to end recruitment at this point.
Data analysis
Continuous data were described by mean and standard deviation (SD), median and inter-quartile range (IQR) and minimum (min) to maximum (max). Proportions were described by counts and proportions expressed as percentages. On the scatter plot, linear regression lines are shown. The estimates of the associations between skin-to-deltoid muscle distance are by mixed linear models with fixed effects for measurement site, sex and arm circumference or BMI; together with associated interaction terms and, as random effects, the individual participants with unstructured variance-covariance correlation structures for the repeated measurements. The difference in paired proportions was estimated by appropriate categorical data models for the proportion of participants with skin-to-deltoid-muscle distances above nominated thresholds.
A threshold of p<0.05 was used to determine if between-site differences in mean skin-to-deltoid-muscle distance were statistically significant. The primary comparisons used the New Zealand definition of injection site as the reference level and compared this to the Australia and USA sites (Hypothesis 1). The secondary analyses included a one degree of freedom test using the ordinal rank of the distance from the acromion process as a predictor to explore if skin-to-deltoid-muscle distance increases as the distance increases (Hypothesis 2). To explore the possibility that the difference in skin-to-deltoid-muscle distance between sites differed by sex (Hypothesis 3), BMI (Hypothesis 4), and arm circumference (Hypothesis 5) were explored using mixed linear models with main effects and interaction terms for each of these possible effect modifying predictors. A comparison of the paired proportions was done by a generalised mixed linear model (Hypothesis 6). SAS version 9.4 was used for statistical analyses.
Ethical approval
This study was approved by the Northern B Health and Disability Ethics Committee (REF: 2022 EXP 12121). All participants provided written informed consent prior to participation.
Results
View Figures 1–2 and Tables 1–3.
There were 29 female and 11 male (n=40) participants with a mean (SD) age of 52.0 (16.3) years. There were no missing data. Demographic and anthropometric data are shown in Table 1. Mean (SD) BMI was 36.8 (5.0) kg/m[[2]] and the mean (SD) skin-to-deltoid-muscle distance for the USA, Australia and New Zealand sites were 13.96 (4.54), 17.94 (6.08) and 20.26 (5.91) mm respectively. Note that a recommended needle length is one that is 5mm greater than the skin-to-deltoid-muscle distance. Using prioritised ethnicity, 90% of participants were European, 5% were Māori, 2.5% were Pasifika, and 2.5% were of Middle Eastern/Latin American/African ethnicity. Respiratory disease was the most common comorbidity, being present in 70%, followed by cardiovascular disease in 27.5% and diabetes in 5% of the study population. This table also shows variability in the distances to injection sites in relation to anatomical landmarks recommended by the Australian and New Zealand guidelines and the summary data of the arm circumference measured at each injection site.
Differences in skin-to-deltoid muscle distance between recommended injection sites
The estimates of the differences between skin-to-deltoid-muscle distance ultrasound measurements between injection sites are displayed in Table 2. There was an interaction between site of measurement of skin-to-deltoid-muscle distance and sex, P-interaction 0.035; the sex-specific differences are also shown. The interpretation is that skin-to-deltoid-muscle distance is larger for the New Zealand than the Australia and USA sites and that these differences between injections sites are in turn larger for men compared to women.
Skin-to-deltoid-muscle distance in relation to anthropometric measurements
Distance from acromion
When the three measurement sites are treated as ordinal scale variables one unit apart in the order of increasing distance from the acromion (USA then Australia then New Zealand sites), the estimate of increase in skin-to-deltoid-muscle distance per “unit” increase in measurement site is 3.1mm (95% CI 2.7 to 3.6).
Arm circumference
Increasing arm circumference was associated with greater skin-to-deltoid-muscle distance for all three injection sites (Figure 2a and Figure 2b) independent of sex (p=0.56), measurement site (p=0.30), or the combination of sex and measurement site (p=0.39). The estimated coefficient (95% CI) was 0.77 (0.51 to 0.97) mm increase in skin-to-deltoid-muscle distance per 1cm increase in arm circumference.
BMI
There was a positive association between skin-to-deltoid-muscle distance and BMI for all three injection sites (Figure 3c and Figure 3d), independent of sex (p=0.33), or measurement site (p=0.27) or the combination of sex and measurement site (p=0.10). The estimated slope (95% CI) was 0.65 (0.40 to 0.90) mm increase in skin-to-deltoid-muscle distance per unit increase in BMI.
Proportions of participants with skin-to-deltoid-muscle distance greater than 20mm and 33mm
The two standard needle lengths in New Zealand are 25mm and 38mm. After allowing for 5mm penetration into the deltoid muscle these needle lengths correspond to skin-to-deltoid-muscle distances of 20mm and 33mm. No participant had a skin-to-deltoid-muscle distance of greater than 33mm at any measurement site, and so based on this sample no participant would require a needle length greater than 38mm. The counts and proportions of participants with a skin-to-deltoid-muscle distance greater than 20mm, and therefore needing a needle length of greater than 25mm, including cross-classification by measurement site, are shown in Table 3. These proportions were 45% for the New Zealand, 40% for the Australian and 15% for the USA measurement sites respectively. The differences in paired proportions (95% CI) were USA versus New Zealand, 30% (15.8 to 44.2), p<0.001, and Australia versus New Zealand, 5% (-1.8 to 11.8), p=0.15.
Discussion
This study has identified that in adults with obesity, defined as a BMI above 30kg/m[[2]], there are marked differences in the skin-to-deltoid-muscle distance measured at three recommended sites for deltoid intramuscular vaccine injection. The magnitude of the differences was of clinical importance, with the New Zealand site having a mean skin-to-deltoid-muscle distance approximately 6mm and 4mm greater than the recommended sites in USA and Australian guidance respectively. Skin-to-deltoid-muscle distance was greater in females, and participants with greater BMI and larger arm circumference were likely to have greater skin-to-deltoid-muscle distances. These findings suggest that when choosing the appropriate needle length to achieve intramuscular injection in a population with obesity, consideration needs to be given to the specific deltoid injection site selected, as well as the sex, BMI and/or the arm circumference of the individual.
The mean skin-to-deltoid-muscle distance (SD) was 14.0mm (4.5), 17.9mm (6.1) and 20.3mm (5.9) at the recommended USA, Australian and New Zealand injection site respectively. These measures can be considered in relation to the standard 25mm needle used for intramuscular injection of vaccine in clinical practice. A threshold of 20mm between skin and deltoid muscle allows 5mm penetration of a standard 25mm needle into the deltoid muscle. Our findings suggest that the proportion of adults with obesity that may require a longer-than-standard (>25mm) needle to achieve intramuscular delivery was 45%, 40% and 15% for the New Zealand, Australian and USA injection sites respectively. In the context of mass vaccination, this suggests that a substantive proportion of adults require longer-than-standard needles for vaccines that require intramuscular delivery, and that this proportion is greater for vaccination guidance recommending injection sites more distal from the acromion such as Australia and New Zealand. This finding highlights the importance of awareness among vaccine administrators and vaccine recipients to consider using or requesting a longer needle to ensure intramuscular vaccine delivery. This is relevant not only for the mRNA vaccines being administered in global efforts to alleviate the burden of the COVID-19 pandemic,[[22]] but also the numerous other vaccines recommended for intramuscular injection.[[11,15]]
Sex is also an important determinant of skin-to-deltoid-muscle distance and may help inform needle length choice. The mean skin-to-deltoid-muscle distance measurements were greater for females, whereas the differences in skin-to-deltoid-muscle distance between the three studied injection sites were larger for males. The current study found a positive association between skin-to-deltoid-muscle distance and each of arm circumference and BMI. The estimated increases in skin-to-deltoid-muscle distance per 1cm in arm circumference, and one unit in BMI were 0.77mm and 0.65mm respectively. For example, a 6.5cm increase in arm circumference and a 7.7 unit increase in BMI correspond to an extra 5mm skin-to-deltoid-muscle difference. Both measurements could be used in practice to predict an individual’s skin-to-deltoid-muscle distance to inform needle length choice, however, measuring one’s arm circumference may be preferred. Arm circumference is quick, non-invasive, easy to learn, does not require a calculation and may provoke less stigmatisation compared to using BMI to predict skin-to-deltoid-muscle distance. These findings support the derivation and use of different sex-based BMI and arm circumference thresholds for recommending standard or longer needle length for intramuscular injection. This study was not large enough to determine thresholds of arm circumference or BMI, with adequate sensitivity and specificity, that might also determine appropriate needle length.
This study shows that skin-to-deltoid-muscle distance increases at injection sites more distal from the acromion process, and closer to the deltoid tuberosity, which is consistent with findings from other clinical studies.[[5,19,20]] Injection sites further from the acromion process reduce the risk of the needle hitting the axillary nerve and posterior circumflex humeral artery.[[19]]
Limitations
Our results may not be generalisable to populations with different age ranges, ethnicities and comorbidities, characteristics that may influence body fat and muscle distributions.[[23]] Our study did not include children and disproportionately included those with respiratory disease comorbidities, which reflects our institution’s participant database. Studies with larger sample sizes in different populations are needed to enable predictive models for an individual’s skin-to-deltoid-muscle distance to be derived. This study had insufficient power to provide estimates of the arm circumference and BMI cut-points where a needle longer than the standard 25mm length is required to ensure intramuscular delivery of a vaccine in the deltoid muscle.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied in a population with obesity. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Summary
Abstract
Aim
Worldwide, immunisation guidelines variably locate the deltoid injection site based on anatomical landmarks. This may influence the skin-to-deltoid-muscle distance and therefore the needle length required to achieve intramuscular injection. Obesity is associated with increased skin-to-deltoid-muscle distance, but it is unknown whether the injection site location chosen in individuals with obesity impacts the needle length required for intramuscular injection. The aim of the study was to estimate the differences in skin-to-deltoid-muscle distance between three different vaccine injection sites recommended by the national guidelines of the United States of America (USA), Australia and New Zealand, in obese adults. The study also explored i) the associations between skin-to-deltoid-muscle distance across the three recommended sites with sex, body mass index (BMI), and arm circumference, and ii) the proportion of participants with a skin-to-deltoid-muscle distance >20 millimetres (mm), in whom the standard 25mm needle length would not ensure deposition of vaccine within the deltoid muscle.
Method
Non-interventional cross-sectionalstudy in a single site, non-clinical setting in Wellington, New Zealand. Forty participants (29 females), aged ≥18years, with obesity (BMI>30 kilograms [km]/m[[2]]). Measurements included distance from acromionto injection sites, BMI, arm circumference, and skin-to-deltoid-muscle distancemeasured by ultrasound at each recommended injection site.
Results
Mean (SD) skin-to-deltoid-muscle distances for USA, Australia and New Zealand sites were 13.96mm (4.54), 17.94mm (6.08) and 20.26mm (5.91) respectively, with a mean (95% confidence interval) for the distance between Australia minus New Zealand -2.7mm (-3.5 to -1.9), P<0.001; and USA minus New Zealand -7.6 mm (-8.5 to -6.7); P<0.001. Skin-to-deltoid-muscle distance was greater in females and was positively associated with BMI and arm circumference. The proportions with a skin-to-deltoid-muscle distance >20 mm were 45%, 40% and 15% for the New Zealand, Australia and USA sites respectively. However, the sample size was relatively small, limiting interpretation in specific sub-groups.
Conclusion
There were marked differences in the skin-to-deltoid-muscle distance between the three recommended injection sites studied. When choosing the required needle length to achieve intramuscular vaccination in obese vaccine recipients, consideration needs to be given to the injection site location, sex, BMI and/or arm circumference, as these factors all influence the skin-to-deltoid-muscle distance. A standard needle length of 25mm may be insufficient to ensure deposition of vaccine into the deltoid muscle in a substantive proportion of adults with obesity. Research is urgently required to determine anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure intramuscular vaccination.
Author Information
Acknowledgements
Correspondence
Correspondence Email
Competing Interests
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Skin-to-deltoid-muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study
Messenger ribonucleic acid (mRNA) vaccines developed to protect against SARS-CoV-2 infection are only approved for intramuscular administration. A needle of sufficient length to penetrate through the skin and subcutaneous tissue is needed to reach the deltoid muscle (the skin-to-deltoid-muscle distance), which is the preferred injection site.
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