I wound up on NewsNation, WGN’s TV news show, last night and was asked about the New York Times article that discussed women having more adverse reactions to the RNA coronavirus vaccines. While I think the New York Times’ article is accurate and covers the topic well, there were a few additional points I believe needed to be made to improve our understanding of this phenomenon.
Not all adverse reactions are the same or caused in the same way. The acute allergic reactions that have been reported occur for very different reasons than the inflammatory reactions that follow the second dose of the vaccine. While both may be accentuated in women, they do so for different reasons.
Allergic reactions have been accentuated in women predominantly for what is thought to be hormonal reasons. Estrogen, the primary female hormone, causes vasodilatation (dilation of the blood vessels). This has been associated with more severe allergic reactions in a number of cases, including peanut allergy. The hormonal effects can potentiate the blood vessel dilatation caused by the allergy.
Women also have a better protective immune system than men. This is based on genetics as many of the genes that control immunity are present on the X chromosome. Because women get two copies of these genes and men only one, men tend to have lesser inflammatory and protective responses.
Since women’s immune systems respond more strongly, they have more significant adverse responses to a second dose of vaccine where you’re trying to “boost” preexisting immunity. These reactions after the second dose of vaccine are a positive sign of protective immunity induced by the vaccine. However, they should be short lived, and if they persist more than 48 hours or worsen over time, you should see your doctor.
Vaccine doses are developed as an average for all male and female subjects. Since women tend to be smaller and often have smaller shoulder muscles (deltoid) where the vaccine is injected, the dose may be proportionally greater for them. This most often results in more severe side effects in women as compared to men, where it may be more likely to lead to inadequate protection.
A final thought is that in the future, studies may identify different vaccine doses for men and women. This could optimize protection in men, while minimizing side effects in women.