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

Periods can be affected by a variety of factors, including stress, thyroid dysfunction, endometriosis or fibroids. For those who have questions about your menstrual cycle, be sure to speak with your physician.

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 on the agency came from women, even though only 61 percent of the vaccines had been administered to women.

It could be that girls 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 larger degree. Were not sure which it is, she said.

If women are in fact having more side effects than men, there might be a biological explanation : Girls and women can create up to twice as many antibodies after getting flu shots and vaccines for measles, mumps and rubella (M.M.R.) and hepatitis A and B, likely due to a mix of factors, such as reproductive hormones and genetic differences.

A study discovered that over almost three decades, women accounted for 80 percent of all adult allergic reactions to vaccines. Likewise 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 printed in the New England Journal of Medicine describing the experiences of people who’d redness, itching and swelling that started 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.