Do menstrual cycles change after COVID vaccination? Another study finds it’s possible

With more than 222 million people in the U.S. fully vaccinated against the coronavirus, some reports, as well as questions, have emerged about the vaccines’ potential effects on the body — including on menstrual cycles.

One dose of a COVID-19 vaccine was found to temporarily alter menstruation and was linked to a slightly longer cycle in a study published in January, McClatchy News previously reported. This work received funding from the National Institutes of Health.

Months later, another study has found menstrual cycles may temporarily change after a person gets a COVID-19 vaccine.

Thousands of people who received two COVID-19 vaccine doses experienced a change in their typical periods, including heavier periods or breakthrough bleeding, according to a study published July 15 in the journal Science Advances. The work also received NIH funding.

Of those with regular periods, 42% said they had heavier bleeding after at least one dose of a COVID-19 vaccine, the study found. In comparison, about 44% reported their flow did not change. Additionally, roughly 14% experienced a mix of a lighter flow or no change.

The study surveyed more than 39,000 fully vaccinated people, between the ages of 18 to 80, who reported they never tested positive for COVID-19. It involved researchers from the Washington University School of Medicine in St. Louis and the University of Illinois at Urbana-Champaign.

Another major finding included how a number of participants who typically do not get their periods, such as those who are postmenopausal, said they experienced breakthrough bleeding, according to the analysis.

Those who reported breakthrough bleeding and do not usually menstruate included 71% of people using birth control, 39% of people using gender-affirming hormones and 66% of postmenopausal people.

In early 2021, “menstruating and formerly menstruating people began sharing that they experienced unexpected bleeding” following COVID-19 vaccination, study authors wrote.

“Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine.”

The ‘most common postvaccination change’

The reports of heavier bleeding from those who have regular periods and breakthrough bleeding from those who do not were called an “increased bleeding phenotype” and “appeared to be the most common postvaccination change” in the study. This is because it was an observed, reported change.

The study was conducted after one of its authors, Kathryn Clancy, of the University of Illinois at Urbana-Champaign’s department of anthropology, experienced an unusually heavy period 10 days after getting her first COVID-19 vaccine dose in early 2021, she told Science.

Clancy described it as “menstrual flooding.”

For the research, Clancy and her colleagues launched a survey on April 7, 2021, and collected participant responses from 39,129 people, who were mostly vaccinated with Pfizer or Moderna’s COVID-19 vaccines, until June 29, 2021. Others were vaccinated with COVID-19 shots including Johnson & Johnson, AstraZeneca and Novavax.

After the survey was shared to Twitter and several other online platforms, participants answered multiple choice and text entry questions about their menstrual cycles and experiences after vaccination.

Of those who regularly menstruate, changes in period flows were noticed within one week after vaccination, eight to 14 days after vaccination and more than two weeks after vaccination, according to the work.

The changes noticed more than a week after vaccination extended “beyond the typical 7 days of closely monitored adverse symptom reporting,” study authors noted.

The researchers wrote that heavier bleeding was more likely for those “who were of non-white race, were Hispanic/Latinx, were older, had a diagnosed reproductive condition, used hormonal contraception, had been pregnant in the past (whether or not they had given birth), were parous, or experienced fever or fatigue after vaccination.”

Findings are evidence to further study trends

Study authors believe their analysis “is the very first characterization of postvaccine menstrual bleeding changes for a gender-diverse sample of pre- and postmenopausal people.”

“The associations described here are not causal but provide evidence to better study these trends further,” they wrote.

Authors also noted that the findings cannot be generalized as the experiences of the general public.

Overall, both increased and breakthrough bleeding were “significantly associated with age, systemic vaccine side effects (fever and/or fatigue), history of pregnancy or birth, and ethnicity,” the study said.

Based on the findings, authors suggest “menstrual changes in the context of what is typical for the vaccinated person” should be considered by health care providers and those who are vaccinated.

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