Intramuscular vaccination needle length: a call to arms
Authors: Marjan Doppen, Ciléin Kearns, Thomas Hills, Mark Weatherall, Richard Beasley
Journal: The Lancet (2024)

When compared with intramuscular administration, subcutaneous administration of vaccines can be associated with a higher frequency and severity of injection site reactions.1Cook IF Subcutaneous vaccine administration—an outmoded practice.Hum Vaccin Immunother. 2021; 17: 1329-1341Crossref PubMed Scopus (28) Google Scholar These reactions are of public health significance because adverse events following immunisation contribute to vaccine hesitancy or refusal, which at a population level can contribute to reduced vaccine uptake and an associated increased prevalence of vaccine-preventable disease.1Cook IF Subcutaneous vaccine administration—an outmoded practice.Hum Vaccin Immunother. 2021; 17: 1329-1341Crossref PubMed Scopus (28) Google Scholar, 2Gowda C Dempsey AF The rise (and fall?) of parental vaccine hesitancy.Hum Vaccin Immunother. 2013; 9: 1755-1762Crossref PubMed Scopus (176) Google Scholar Immunogenicity of some vaccines, such as hepatitis B and influenza vaccines, is reduced if given via a subcutaneous injection rather than an intramuscular injection.1Cook IF Subcutaneous vaccine administration—an outmoded practice.Hum Vaccin Immunother. 2021; 17: 1329-1341Crossref PubMed Scopus (28) Google Scholar The standard needle length for deltoid intramuscular injection is 25 mm. Unfortunately, for up to 45% of adults with obesity, the standard needle can be inadequate to ensure intramuscular deposition of vaccine, with the specific deltoid injection site location also influencing the depth the needle must penetrate to reach muscle.3Doppen M Black M Braithwaite I et al.Skin-to-deltoid muscle distance at three recommended sites for intramuscular vaccination in a population with obesity: an observational study.NZMJ. 2023; 136: 67-76PubMed Google Scholar The global prevalence of obesity is high and increasing, so evidence-based advice on how to select the optimal needle length for people receiving intramuscular vaccines is needed. However, there is only a small evidence base to reliably identify vaccine recipient characteristics that can be used to choose individuals who might require a longer needle, and an absence of specific recommendations in immunisation guidelines. For example, the UK Health Security Agency Green Book recommends that a longer needle length (eg, 38 mm) might be required in so-called larger adults, and that individual assessments for needle length should be made.4UK Health Security AgencyThe Green Book: immunisation against infectious disease: Chapter 4: Immunisation procedures.https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/147915/Date: 2013Date accessed: August 29, 2023Google Scholar However, this advice does not quantify at what point an adult, or their arm, is considered large enough that the 38 mm needle is required. This issue has been investigated in New Zealand by ultrasound measurement of the skin to deltoid muscle distance, reporting that both arm circumference and BMI cut-points provide practical ways to choose needle length to increase the chance of successful intramuscular vaccination.5Doppen M Kearns C Weatherall M et al.When should a longer needle be used for intramuscular injection in obese patients? A combined analysis of New Zealand data.Vaccine. 2023; 41: 2690-2695Crossref PubMed Scopus (2) Google Scholar On the basis of the recommended New Zealand deltoid vaccination site, and assuming that a minimum penetration of 5 mm into muscle is required, an arm circumference of greater than or equal to 35 cm for men and greater than or equal to 30 cm for women are cut-points in which a longer 38 mm needle would ensure intramuscular deposition of vaccine. However, the New Zealand findings might not apply to other populations that could have different recommended intramuscular deltoid vaccination sites, or a different population composition of ethnicities with different body habitus characteristics. Therefore, research is urgently required to establish anthropometric measurement cut-points that can be used to enable appropriate needle length selection to ensure optimal vaccine delivery in other populations. RB has received institutional research funding from the Health Research Council of New Zealand and the Ruth Maud Ring Spencer Estate for the research programme: MRINZ IM vaccination needle length. All other authors declare no competing interests.