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COVID vaccines may briefly change your menstrual cycle, but you should still get one

Licensed practical nurse Yokasta Castro, of Warwick, R.I., draws a Moderna COVID-19 vaccine into a syringe. The vaccines have now been linked to minor changes in menstruation, but are still considered safe.
Steven Senne
Licensed practical nurse Yokasta Castro, of Warwick, R.I., draws a Moderna COVID-19 vaccine into a syringe. The vaccines have now been linked to minor changes in menstruation, but are still considered safe.

A new scientific study shows that vaccination can cause changes to the timing of menstruation. But it also shows the effects are temporary, more akin to a sore arm than a serious adverse event.

"I think it's reassuring and also validating," says Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University in Portland, Ore. who led the study.

The work appeared Thursday in the journal Obstetrics and Gynecology. Edelman and other experts stress that individuals should get vaccinated, because the risks from COVID-19 remain high.

Clinical trials for the COVID vaccines looked for side-effects like headaches or fever, but when it came to reproductive health, the main focus was on pregnancy, not menstruation.

"The menstrual cycle is like the stepsister that gets ignored," Edelman says. "It's considered unimportant in the grand scheme of things, but it actually really is important to people day-to-day."

And many people did notice changes to their menstrual cycles. A survey conducted by anthropologists found numerous reports of unusually heavy flows and even breakthrough bleeding among some people who hadn't menstruated in years.

Anti-vaccine activists capitalized on other anecdotal reports from social media–using them to make unfounded claims that the vaccines were being used to spread infertility and ultimately depopulate the earth.

Researchers take a closer look at menstruation

Clinical trials and other studies have already established the vaccines are safe and effective for pregnant women, but the rumors that surrounded menstruation made the National Institutes of Health decide to take a closer look.

"There was a need to be able to counsel women on what to expect," says Dr. Diana Bianchi, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which funded the work.

Edelman's team took data from a popular app known as "Natural Cycles," which people can use to track their menstrual cycles. Looking at data from 3,959 individuals, they were able to see a small shift in the time between bleeding.

"We see a less-than-one-day change in their menstrual cycle length with vaccination," Edelman says.

In other words, people who were vaccinated experienced — on average — a slightly longer menstrual cycle around the time of their first and second doses.

"It's really nothing to get alarmed about," Bianchi says. Nevertheless, she adds, it does prove that the vaccines are affecting menstruation independently of other possible factors such as pandemic stress.

"I think the beauty of the study is that it does affirm what individuals were reporting," she says.

The immune and reproductive systems are linked

Researchers still aren't exactly sure why the changes occur. Edelman says the immune system and the reproductive system are linked, and some signaling immune cells also can affect the body's natural clock. Given that relationship, it's entirely possible that triggering the immune system through vaccination might change a person's cycles. But whatever's happening seems to be short-lived.

"We haven't seen anything that's concerning regarding fertility or pregnancy in terms of vaccination," she says. Moreover, she says, COVID-19 itself can have a profound effect on health.

"The risk of COVID-19 disease in pregnant women is incredibly serious," said Edelman, who has watched pregnant women end up in intensive care because they're not vaccinated.

Bianchi says that the NIH should have more data on how vaccines alter menstruation in the near future. Last summer, the agency funded five studies including Edelman's work, and the results of the other researchers should provide more data. Bianchi hopes that the additional research can look at a more diverse population than those that use the app, and also start to nail down other reported side-effects, including heavy periods and breakthrough bleeding.

Ultimately, Edelman thinks changes to menstruation probably should be added as a possible minor side-effect of the vaccines, along with headaches and sore arms.

"I think because it rises to a level of importance to the public," she says, "it's something that should be included."

Copyright 2023 NPR. To see more, visit https://www.npr.org.

Geoff Brumfiel
Geoff Brumfiel works as a senior editor and correspondent on NPR's science desk. His editing duties include science and space, while his reporting focuses on the intersection of science and national security.