COVID-19 shots affected menstrual cycles, new research says

Vaccines temporarily caused heavier bleeding in nearly half of women surveyed

Nearly half of the participants of a recent study who were menstruating regularly at the time of the survey reported heavier bleeding during their periods after receiving the COVID-19 vaccine. Others who did not typically menstruate — including transgender men, people on long-acting contraceptives and postmenopausal women — also experienced unusual bleeding.

The new study — the largest to date — expands on research that has highlighted the temporary effects of COVID-19 vaccines on menstrual cycles but until now focused primarily on cisgender women who menstruate.

Although the vaccines have largely prevented deaths and severe disease with few reported side effects, many medical experts initially brushed aside concerns when women and gender-diverse people started reporting erratic menstrual cycles.

To get a better sense of these post-vaccination experiences, researchers at the University of Illinois at Urbana-Champaign and Washington University School of Medicine in St. Louis distributed an online survey in April 2021 to thousands of people across the globe. After three months, the researchers collected and analyzed more than 39,000 responses from individuals between the ages of 18 and 80 about their menstrual cycles. All the survey respondents had been fully vaccinated — with the Pfizer-BioNTech, Moderna, Johnson & Johnson vaccines or another that had been approved outside the United States. And to the best of their knowledge, the participants had not contracted COVID-19 before getting vaccinated.

The research, published Friday in the journal Science Advances, shows that 42 percent of people with regular menstrual cycles experienced heavier bleeding after vaccination, while 44 percent reported no change, and 14 percent reported lighter periods. Additionally, 39 percent of respondents on gender-affirming hormone treatments, 71 percent of people on long-acting contraceptives and 66 percent of postmenopausal women experienced breakthrough bleeding after one or both of their shots.

“I think it’s important that people know this can happen, so they’re not scared, they’re not shocked, and they’re not caught without supplies,” said Katharine Lee, a biological anthropologist at the Washington University School of Medicine and the study’s first author.

Lee cautioned, however, that the study did not compare the results with a control group of people who did not get vaccinated. And it is possible that people who observed changes in their cycles after vaccination may have been more likely to participate in the survey. Still, the findings line up with smaller studies that have reported menstrual changes after vaccination with more robust controls.

Importantly, the new study also found that some demographics may be more likely to experience menstrual changes, and the study may help them be better prepared, Lee said. A heavier menstrual flow was more likely for those who were older, for instance. Survey respondents who used hormonal contraception, had been pregnant in the past or had been diagnosed with a reproductive condition like endometriosis, fibroids or polycystic ovarian syndrome were also more likely to have heavier bleeding during their periods. People who identified as Hispanic or Latino tended to report heavier bleeding, too.