Largest Study Yet Looks at Period Changes After Vaccine

More than half of survey respondents with regular periods before receiving a COVID-19 vaccine described changes in their menstrual cycle afterward. The largest study on the issue to date found that 44% of 16,000 respondents with regular periods before vaccination experienced no change in their menstrual cycle afterward, per Science. But 42% described heavier bleeding and 14% described lighter bleeding, reports the New York Times. The study is based on responses from 39,000 people, including post-menopausal women and transgender men. None had COVID before their vaccination, to the best of their knowledge. The study also found that 71% of people on long-acting contraceptives, 66% of post-menopausal women, and 39% of people on gender-affirming hormone treatments reported unexpected, or "breakthrough," bleeding.

Though the research adds to evidence that vaccines for COVID-19, typhoid, Hepatitis B, and HPV affect periods, per Time, it doesn't definitively connect the changes to the vaccine. The hormones that regulate the menstrual cycle can be affected by stress, illness, weight changes, calorie intake, exercise, and more, and there was no control group of unvaccinated people for comparison. People who experienced changes in their menstrual cycle after vaccination may have also been more likely to participate in the 2021 survey. Still, "I think it's important that people know this can happen, so they're not scared" or "shocked," Tulane University biological anthropologist Katharine Lee, lead author of the study published Friday in Science Advances, tells the Times.

She notes older people, Hispanics, those who had a prior pregnancy, and those diagnosed with certain reproductive conditions were more likely to experience heavier bleeding. Though it's possible vaccines affect the lining of the uterus, which has been linked to the immune system, minor changes in a regular menstrual cycle generally aren't a concern. However, significant changes warrant further investigation. And anyone with breakthrough bleeding should check in with a doctor, researchers say. They're now completing a follow-up survey asking about when menstrual cycles normalized, per Science. "We suspect that for most people the changes associated with COVID-19 vaccination are short-term," Lee says in a release. (More menstruation stories.)