While the COVID-19 vaccines have made a dramatic difference in controlling the pandemic, researchers are still learning about the ways that the shots may affect people’s health.
One of those questions centers around how the vaccines might impact menstrual cycles. Reports of women experiencing changes in their cycles—either in the intensity and frequency of bleeding, or the length of their periods—have prompted more rigorous investigations of how COVID-19 vaccines may be affecting cycles.
In the latest study, published in the BMJ, researchers led by Rickard Ljung, professor of epidemiology and physician specialist at the Swedish Medical Products Agency (the country’s equivalent to the U.S. Food and Drug Administration), looked at medical records from nearly 3 million Swedish women, ages 12 to 74. They focused on those who had sought care from health care professionals for menstrual changes from Dec. 2020 to Feb. 2022 and were able to see when the women were vaccinated and how long after their immunization they sought care.
That analysis found no strong association between COVID-19 vaccines and changes in menstruation—or at least bleeding events that were severe enough for women to seek medical care.
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The strongest association was among post-menopausal women. After a third dose (the booster) of either mRNA vaccine, made by Moderna or Pfizer-BioNTech, women had a 28% increased risk of bleeding in the first week after vaccination, and about a 25% increased risk of bleeding through the next three months, compared to before their vaccination. Ljung says that pattern doesn’t necessarily suggest an association between the two events, especially since a similar increase was not seen among premenopausal women.
Ljung says that the association among post-menopausal women could be confounded by a number of factors, including the fact that they might be more attuned to bleeding events and more likely to see a physician than younger women, who may attribute any bleeding to irregular periods. Also, given that the data captured doctor’s visits, it’s unlikely that women would have been able to get an appointment with a physician within a week if they developed bleeding after getting vaccinated. “That’s a very short time to get that kind of appointment,” says Ljung. The pattern, he says, doesn’t support a possible causal connection between vaccination and changes in menstruation, and it’s likely the association seen among post-menopausal women might be reflecting other factors.
The results don’t necessarily contradict the growing number of reports of changes in menstruation following COVID-19 vaccination. Periods can be affected by any vaccine, but the current study shows that those changes might not be serious enough to require medical attention. “It’s a bit reassuring that there doesn’t seem to be an increased risk of more severe menstrual disturbances, bleeding, or other changes after vaccination,” says Ljung.
Still, doctors don’t fully understand why immunization against COVID-19 could affect menstrual cycles, so more research is needed to appreciate how developing immunity against SARS-CoV-2 affects health.
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Contributor: Alice Park