Researchers struggle to understand potential tie between COVID-19 vaccines, menstrual changes: 6 things to know

Thousands of Americans have reported unexpected menstrual cycle changes around the time they received a COVID-19 vaccine, though a causal link hasn't been scientifically established between the shot and the reported changes, according to NPR

Kate Clancy, PhD, human reproductive ecologist and associate professor of anthropology at the University of Illinois Urbana-Champaign, and Katharine Lee, PhD, biological anthropologist at Washington University School of Medicine in St. Louis, have collected responses from more than 140,000 people reporting a change in their periods after vaccination.  

Six things to know:

1. "We started hearing a lot about breakthrough bleeding, from people on long-acting reversible contraception, people on gender-affirming hormones, and postmenopausal people who were years and years out from their last period, sometimes decades out," Dr. Lee told NPR, noting that individuals typically describe only brief disruptions to menstruation.  

2. Even in large numbers, the individual stories don't prove that COVID-19 vaccines are causing menstruation changes. The stories represent just a small portion of the millions vaccinated, and menstrual cycles vary widely and are affected by many factors. Most women — including those in the clinical trials — haven't experienced changes to menstruation, according to statements from Pfizer, Moderna and Johnson & Johnson, as cited by NPR. 

3. The FDA and all three vaccine manufacturers said they haven't recorded any evidence that the vaccine causes menstrual irregularities or any related health problems. The American College of Obstetricians and Gynecologists still highly recommends the COVID-19 vaccine and said there's no reason to schedule vaccination around menstrual events.

4. The early studies didn't ask much about reproductive health, aside from questions around pregnancy, underscoring a blind spot in how clinical research was conducted. "It seeds doubt," said Dr. Lee. "It makes people feel like their bodies were not considered in part of the trial."

5. Both researchers said they are "pro-vaccine," but are troubled by some people reporting that their concerns were dismissed by physicians, which may be happening in part because menstruation changes aren't officially listed as a possible side effect.

6. The CDC is reviewing reports from its database to see if it's possible to detect how the vaccine may affect menstruation, an agency spokesperson told NPR.

 

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