The importance of injecting vaccines into muscle

Different patients need different needle sizes

Most vaccines should be given via the intramuscular route into the deltoid or the anterolateral aspect of the thigh. This optimises the immunogenicity of the vaccine and minimises adverse reactions at the injection site. Recent studies have highlighted the importance of administering vaccines correctly.1^–3 Clinical practice needs to reflect considerations about the right length and gauge of needles used to ensure that those vaccinated get the immunological benefit of the vaccines without local side effects.

Injecting a vaccine into the layer of subcutaneous fat, where poor vascularity may result in slow mobilisation and processing of antigen, is a cause of vaccine failure1—for example in hepatitis B,2 rabies, and influenza vaccines.3 Compared with intramuscular administration, subcutaneous injection of hepatitis B vaccine leads to significantly lower seroconversion rates and more rapid decay of antibody response.1

Traditionally the buttocks were thought to be an appropriate site for vaccination, but the layers of fat do not contain the appropriate cells that are necessary to initiate the immune response (phagocytic or antigen-presenting cells). The antigen may also take longer to reach the circulation after being deposited in fat, leading to a delay in processing by macrophages and eventually presentation to the T and B cells that are involved in the immune response. In addition, antigens may be denatured by enzymes if they remain in fat for hours or days. The importance of these factors is supported by the findings that thicker skinfolds are associated with a lowered antibody response to vaccines. 1 2

Serious reactions to intramuscular injections are rare; in one series of 26 294 adults, of whom 46% had received at least one intramuscular injection, only 48 (0.4%) had a local adverse effect.4 However, subcutaneous injections can cause abscesses and granulomas. 1 5 6 Muscle is probably spared the harmful effects of substances injected into it because of its abundant blood supply.5 Adipose tissue, having much poorer drainage channels, retains injected material for much longer and is therefore also more susceptible to its adverse effects.5 In the case of vaccines in which the antigen is adsorbed to an aluminium salt adjuvant—such as those for hepatitis A, hepatitis B, and diphtheria, tetanus, and pertussis vaccines—the intramuscular route is strongly preferred because superficial administration leads to an increased incidence of local reactions such as irritation, inflammation, granuloma formation, and necrosis. 2 7 8

The injection technique and needle size both determine how deep a substance is injected. Injection technique involves stretching the skin flat before inserting the needle or pinching a fold of skin before injection, which may necessitate the use of longer needles. To make sure the needle reaches the muscle and that vaccine does not seep into subcutaneous tissue the decision on the size of the needle and injection site should be made individually for each person. It should also be based on the person's age, the volume of material to be administered, and the size of the muscle.9

In a recent study, the thickness of the fat pad above the deltoid muscle of the upper arm was measured in 220 adults (healthcare workers presenting for hepatitis B immunisation) using high frequency ultrasonography.1 A wide variation exists in thickness of the deltoid fat pad, with women having significantly more subcutaneous fat than men. A standard 5/8 inch (16mm) needle would not have achieved sufficient penetration for true deltoid intramuscular injection in 17% of men and nearly 50% of women in the study population.1 For men weighing 59-118 kg and women of 60-90 kg it may be safer to use a 1 inch (25mm) needle. A woman over 90 kg may need a 1.5 inch (38mm) needle.

Healthcare professionals may hesitate to use longer needles on the grounds that they are likely to cause the patient more discomfort. However, skeletal muscle has a poor supply of pain fibres compared with skin and subcutaneous tissue.10

Consideration should be given to needle gauge.11 A wider bore needle ensures that the vaccine is dissipated over a wider area, thus reducing the risk of localised redness and swelling.12

A standard size of needle will not guarantee successful intramuscular injection in all people. When intramuscular vaccine administration is needed to ensure optimal immunogenicity and minimise local reactions, a selection of non-fixed needles (pre-filled syringes that may be provided with a needle fixed on the barrel) should be available to allow healthcare professionals to select a length and gauge of needle appropriate to each patient.