A new analysis of reports from more than 35,000 people offers the most comprehensive assessment so far of menstrual changes experienced by pre- and post-menopausal individuals in the first two weeks after receiving the COVID-19 vaccine. Published in the journal Science Advances, the study adds to the evidence that significant numbers of people experience this unexpected side effect.

“Menstruating and formerly menstruating people began sharing that they experienced unexpected bleeding after being administered a COVID-19 vaccine in early 2021,” the scientists who led the study wrote. Because vaccine trials typically do not ask about menstrual cycles or bleeding, this side effect was largely ignored or dismissed.

Doctors who weighed in on the subject after hearing early reports about post-vaccination menstrual changes were often dismissive of patients’ concerns, said Kathryn Clancy, a professor of anthropology at the University of Illinois Urbana-Champaign who led the research with Katharine Lee, an anthropology professor at Tulane University. Some clinicians said it was unclear how a vaccine could trigger such changes.

However, other vaccines — including those for typhoid, Hepatitis B and HPV — are sometimes associated with changes in menstruation, Clancy said. These side effects are believed to be tied to an uptick in immune-related inflammatory pathways and are less likely to be driven by hormonal changes.

“We suspect that for most people the changes associated with COVID-19 vaccination are short-term, and we encourage anyone who is worried to contact their doctor for further care,” Lee said. “We want to reiterate that getting the vaccine is one of the best ways to prevent getting very sick with COVID, and we know that having COVID itself can lead not only to changes in periods, but also hospitalization, long COVID and death.”

The researchers used a survey to query people about their experiences after vaccination. Launched in April 2021, the survey asked for demographic and other information but focused on respondents’ reproductive history and experiences regarding menstrual bleeding. The team downloaded the data from the surveys on June 29, 2021. Only those who had not been diagnosed with COVID-19 were included in the analysis, as COVID-19 itself is sometimes associated with menstrual changes. The study also excluded data from people 45 to 55 years old to avoid confounding the results by including menstrual changes associated with perimenopause.

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