Women and the Covid-19 Vaccine: What You Need to Know

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Kathryn Clancy, an associate professor of anthropology at the University of Illinois, generated hundreds of responses on Twitter after saying that her period was heavier than usual after her first dose of the Moderna vaccine. She is now collaborating with Katharine Lee, a postdoctoral research scholar at Washington University in St. Louis, to survey women on short-term vaccine side effects related to the menstrual cycle. Their online survey has been available for less than a week and has so far drawn more than 19,000 responses, Dr. Lee said on Wednesday.

Periods can be affected by a multitude of factors, including stress, thyroid dysfunction, endometriosis or fibroids. If you have questions about your menstrual cycle, be sure to speak with your doctor.

A study by the Centers for Disease Control and Prevention, published in February, examined the Pfizer-BioNTech and Moderna vaccines and found that 79 percent of the side effects reported to the agency came from women, even though only 61 percent of the vaccines had been administered to women.

It could be that women are more likely to report side effects than men, said Dr. Sabra L. Klein, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. Or, she added, women might be experiencing side effects to a greater degree. “We’re not sure which it is,” she said.

If women are in fact having more side effects than men, there might be a biological explanation: Women and girls can produce up to twice as many antibodies after receiving flu shots and vaccines for measles, mumps and rubella (M.M.R.) and hepatitis A and B, probably because of a mix of factors, including reproductive hormones and genetic differences.

A study found that over nearly three decades, women accounted for 80 percent of all adult allergic reactions to vaccines. Similarly, the C.D.C. reported that most of the anaphylactic reactions to Covid-19 vaccines, while rare, have occurred among women.

And in a letter published in the New England Journal of Medicine describing the experiences of people who had redness, itching and swelling that began four to 11 days after the first shot of the Moderna vaccine, 10 of the 12 patients were women. It is not clear, however, whether women are more prone to the problem.