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

Kathryn Clancy, an associate professor of anthropology at the University of Illinois, made hundreds of responses on Twitter after saying that her periods were heavier than usual after her first dose of the Moderna vaccine. She is now working with Katharine Lee, a postdoctoral fellow at Washington University in St. Louis, to interview women about short-term side effects of vaccines related to the menstrual cycle. Her online survey has been available for less than a week and has received more than 19,000 responses to date, said Dr. Lee on Wednesday.

A variety of factors can affect periods, including stress, thyroid dysfunction, endometriosis, or fibroids. If you have any questions about your menstrual cycle, be sure to talk to your doctor.

A study by the Centers for Disease Control and Prevention published in February looked at the Pfizer-BioNTech and Moderna vaccines and found that 79 percent of the side effects reported to the agency were from women, although only 61 percent of the vaccines were administered to women.

Women could be more likely to report side effects than men, said Dr. Sabra L. Klein, Professor of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health. Or, she added, women may experience side effects to a greater extent. “We’re not sure what it is,” she said.

If women actually experience more side effects than men, there may be a biological explanation: women and girls can make up to twice as many antibodies after flu shots and vaccines for measles, mumps, and rubella (MMR), and hepatitis A and B, probably due to a mixture of factors including reproductive hormones and genetic differences.

One study found that women accounted for 80 percent of all adult allergic reactions to vaccines in nearly three decades. Similarly, the CDC reported that most anaphylactic reactions to Covid-19 vaccines, although rare, occurred in women.

In a letter published in the New England Journal of Medicine describing people’s experiences with redness, itching, and swelling that began four to eleven days after the first shot of the Moderna vaccine, 10 of the 12 patients were women. However, it’s not clear if women are more prone to the problem.

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