We Finally Know More About How COVID Vaccines May Temporarily Change Your Period

As the COVID-19 vaccines first rolled out in the United States, an unanticipated, anecdotal side effect started to emerge: Many people speculated that the vaccine seemed to have an effect on their menstrual cycle.

Studies that examined whether those anecdotes could be explained by science were sparse and small—which is not surprising, given the slow pace of research on menstruation within vaccine trials. Then, in July, the largest study of its kind published in the journal Science Advances added some support to the claims people were sharing online. Here’s what we know so far about how the COVID vaccines may affect a person’s period and why more research is still needed.

How the vaccine may affect those who menstruate regularly

To start with a caveat: Even with the newest Science Advances study, Alyssa Dweck, MD, FACOG, an ob-gyn in New York, emphasizes that most of the evidence we have on the COVID vaccines’ potential effects on periods is still anecdotal. Meaning, if there is a clear cause-and-effect link between the vaccines and menstrual changes, it has not been scientifically established yet.

However, the Science Advances study does suggest there could be a correlation between the vaccine regimen and temporary changes in a person’s menstrual cycle. For the study, researchers surveyed more than 35,000 people between the ages of 18 and 80. A majority of respondents received either the Pfizer-BioNTech or Moderna two-dose series, so these vaccines were the focus of the study’s analyses. Respondents were also vaccinated with the Johnson & Johnson, Novavax, and AstraZeneca vaccines.

According to the study, 42% of people surveyed who regularly menstruate reported having heavier periods after receiving a COVID vaccine, while 44% reported experiencing no changes to their periods at all.

“In my practice, I’ve definitely seen people have changes in their menstruation—whether it’s the onset, the duration, or the heaviness [of their period]—after the vaccines,” Dr. Dweck tells SELF. “But it seems to be transient. Or, in other words, it’s not every cycle after the fact, but really more like a one and done type of situation.”

Recent studies support Dr. Dweck’s observations. A survey of nearly 4,000 people published in The BMJ in January 2022 found that those who experienced longer or heavier periods returned to their typical flow within two menstrual cycles post-vaccination. And a 2022 study out of Norway published by the Norwegian Institute of Public Health similarly found that people’s periods generally returned to normal within two to three cycles. In response to these studies, The American College of Obstetricians and Gynecologists updated its COVID-19 vaccine FAQ page to reflect that there may be a “small, temporary change” in a person’s menstrual period after vaccination.

How the vaccine may affect those who don’t menstruate regularly

According to the Science Advances study, changes in periods were more widely reported in those who don’t have a typical period. Breakthrough bleeding (spotting that occurs when a period is not expected) after a COVID vaccine was reported by 71% of people on long-acting reversible contraception (LARC), 39% of people taking gender-affirming hormones, and 66% of postmenopausal people.