Since vaccines began distribution at the end of 2020, the side effects of receiving one have been well documented. People have reported a range of side effects after receiving their vaccination, including pain, redness, and swelling at the injection site. Side effects also include muscle pain, headache, nausea, fatigue, chills, and fever. Reports show that these symptoms are more commonly experienced by women than men and that symptoms experienced by women are often more severe.
The CDC report analyzed data from the first 13.7 million COVID-19 vaccine doses distributed in the United States. It found that, despite only 61.2% of the vaccines going to women, 79.1% of reported side effects were experienced by women.
Though anaphylactic reactions to the COVID vaccines are rare, most of them occurred among women. Nineteen people had anaphylactic reactions to the Moderna vaccine, all of them female. Of the 47 people who suffered from anaphylaxis following a Pfizer vaccination, 44 of them were female.
Dr. Sabra Klein, a microbiologist and immunologist from the Johns Hopkins Bloomberg School of Public Health, noted that this trend is common among vaccines. “I am not at all surprised,” Klein told the New York Times. “This sex difference is completely consistent with past reports of other vaccines.”
Previous studies have shown that women are up to four times more likely than men to experience allergic reactions to vaccines. The reactions experienced are typically mild.
Why are women more likely than men to experience side effects?
A number of potential reasons have been given to explain this trend. Men are commonly less likely to seek medical assistance when ill or injured, so it follows that they may be less likely to report side effects. More concretely, researchers say biology plays a major role, according to the New York Times.
“The female immune response is distinct, in many ways, from the male immune response,” Eleanor Fish, an immunologist at the University of Toronto, told the Times.
Women’s immune systems typically produce far more antibodies in response to vaccines. Researchers believe hormones may play a role. Hormones like estrogen, progesterone, and testosterone influence the way immune cells function.
In response to the flu vaccine, for example, exposure to estrogen prompts immune cells to produce extra antibodies. Testosterone does nearly the opposite, due, in part, to its suppression of the body’s natural production of immune chemicals called ‘cytokines.’ The flu vaccine is often less effective among men with high levels of testosterone.
The difference in immune responses between men and women plays a big role in how the COVID-19 vaccine affects us and what side effects we experience. Women also commonly need smaller doses to reach the same effect, as they metabolize drugs differently than men. The COVID-19 vaccine doses are all the same size, regardless of gender, which may mean that women are getting more of the vaccine than they need.
Regardless of why this trend is occurring, experts say physicians and healthcare providers should discuss it with their female patients ahead of time. “I think that there is value to preparing women that they may experience more adverse reactions,” Klein said. “That is normal, and likely reflective of their immune system working.”
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