The Royal College of Paediatricians states that ‘the administration of a vaccine should ensure the attainment of maximum immunity, with the least possible harm’.
Few would disagree with this statement, yet there has been little research to guide the practical aspects of vaccine delivery despite the importance of immunisation for children’s health.
With needles of different sizes available, practitioners have questioned whether it is the length or gauge of the needle that is most important for correct vaccine delivery?
Although over four million injected vaccines are given to UK infants each year, evidence-based guidance on needle size for intramuscular delivery is lacking.
The width (gauge) of a needle is indicated by hub colour. The latest DoH literature recommends a 25mm long blue hub (23 gauge) needle, but many practitioners prefer to use a 16mm orange hub (25 gauge) needle.
Some local policies stipulate use of the larger diameter 23 gauge needle, drawing on anecdotal evidence that this enables the vaccine to dissipate over a wider space, reducing risks of localised redness and swelling.
Others advise that a 16mm needle adequately reaches the anterolateral thigh muscle when inserted at 90° and using the intramuscular delivery technique recommended by the WHO. WHO, however, recommends a 25mm needle for infant immunisations.
These uncertainties have arisen because of insufficient data to define best practice.
A narrower needle is presumed to be preferable for delivery of non-viscous infant immunisations, because the narrow width may minimise injection trauma and reduce pain at the injection site. However, some guidelines recommend the wider gauge to introduce vaccines under lower pressure, facilitating a slower and steadier delivery that allows muscle to accommodate the vaccine more easily than would be the case if inserted with a narrow needle.
Anecdotally, the wider gauge was also thought to reduce vaccine reactogenicity, enabling the vaccine to dissipate over a wider space and reducing localised swelling.
While a finer gauge is important for intradermal delivery of vaccines such as BCG, which needs to be deposited between the layers of the skin, it is not clear whether gauge is important for intramuscular administration.
However, needle length is likely to be crucial in determining whether a vaccine is delivered correctly. The WHO describes an intramuscular injection as one in which the skin is stretched flat between thumb and forefinger to optimise deep insertion, the needle is inserted at a 90° angle to the skin and the needle length must be long enough to reach deep into the muscle.
Alternatively, a subcutaneous injection technique involves the skin at the injection site being bunched up to aid insertion into fatty tissue, the needle being inserted at a 45° angle to the skin and with a shorter needle length being used to reduce the chance of inadvertently delivering vaccine into muscle.
Correct choice of a needle length is therefore important to ensure intramuscular delivery. Would it matter, however, if infant vaccines were delivered into subcutaneous fat?
Subcutaneous delivery is advantageous for drugs that require a slower method of absorption, because it places the injection into the adipose tissue layer just below the epidermis and dermis, and provides a slow, sustained release into the capillary network. This is preferable for some vaccines, such as MMR, and these vaccines should ideally be delivered subcutaneously.
MMR can also be given into the muscle, as stated within the summary of product characteristics.
However, for vaccines containing aluminium adjuvant, the intramuscular route is preferable. Adjuvants are substances that help stimulate an immune response, and mineral salts, such as aluminium hydroxide, aluminium phosphate, alum (potassium aluminium sulphate) or mixed aluminium salt, are adjuvants that are known to increase the body’s immune response to some vaccines.
Adjuvants are also thought to decrease the toxicity of antigens and to provide stability to some vaccine components.
Vaccines containing adjuvants are thought to be more reactogenic when given subcutaneously. When aluminium adjuvants were discovered in the 1920s they were known to cause severe local reactions and to increase the frequency of injection site abscesses.
Fortunately, subsequent studies revealed the incidence of abscess formation and local reactions could be reduced with delivery deep into muscle. As a result, immunisers were told to avoid subcutaneous delivery of alum precipitated vaccines and a recommendation was made for such vaccines to be delivered into muscle. Although whole-cell pertussis vaccine is no longer used within the UK, there have been recent calls for practitioners to ensure use of the correct needle size to ensure delivery of vaccines into muscle.
While the angle of injection, depth of needle insertion and whether injection site tissue is bunched or stretched also contributes to perfect delivery, use of correct needle length increases the likelihood of intramuscular delivery. Because of this, needle length must be more important than gauge.
Recently our research group demonstrated that use of a 25mm, rather than 16mm needle, reduced rates of local reactions following DPT/Hib vaccine in two, three and four month-old infants. Our study also suggested that vaccine immunogenicity may be improved by use of a longer needle.
In addition, a comparison between needles of the same length but different gauge showed no difference in the incidence of local reactions, thereby contradicting some of the anecdotal evidence providing support for the importance of needle gauge.
So, to answer the question posed at the beginning of this article, I believe there is increasing evidence that needle length, rather than gauge, is more important for infant immunisation.
Linda Diggle is principal research nurse at the Oxford Vaccine Group, University of Oxford