This week’s data gap comes courtesy of a Dr GFP – that is, an actual medical doctor, not me being the world’s worst DJ. Anyway, this Real Dr GFP drew my attention to this study
which looked at the importance of needle size when it comes to administering vaccines.
Historically, apparently, we thought the best place to administer vaccines was your bum. But then we discovered that layers of fat were not great for initiating an immune response, because they have the wrong cells.
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 failure —for example in hepatitis B, rabies, and influenza vaccines. Compared with intramuscular administration, subcutaneous injection of hepatitis B vaccine leads to significantly lower seroconversion rates and more rapid decay of antibody response.
The antigen also seems to hang around longer in fat cells, which can make adverse effects more likely.
…subcutaneous injections can cause abscesses and granulomas. Muscle is probably spared the harmful effects of substances injected into it because of its abundant blood supply. Adipose tissue, having much poorer drainage channels, retains injected material for much longer and is therefore also more susceptible to its adverse effects.
Basically, for maximum vaccine efficacy and minimum adverse reactions at the injection site, the needle should be intramuscular.
And this is where we run into problems. As readers of Invisible Women may remember, women tend to have higher fat and lower muscle distribution than men, and, according to this study, the shoulder area where vaccines are commonly injected is no different:
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.
Obviously, there are many factors that are likely playing into these sex discrepancies, but it’s interesting isn’t it?
Incidentally, a friend of mine who was a volunteer Covid vaccinator told me that in her training she was told to use a longer needle for overweight people, but there was no advice about needle length for women.