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COVID-19 vaccines impact some people's periods temporarily, study confirms

Researchers say that informing people their periods can be impacted "is incredibly important to make sure people are prepared and not surprised."

When people started getting the COVID-19 vaccines last spring, some noticed an unexpected side effect — their periods were heavier, started earlier or lasted longer.

Researchers Katharine Lee, Ph.D., and Kathryn Clancy, Ph.D., heard anecdotally that people experienced changed menstruation and launched a survey to capture post-vaccine period experiences. The results, published in the journal Science Advances, indicate the COVID-19 vaccine does change menstruation for some.

“This is indeed happening,” Lee, an assistant professor in the anthropology department at Tulane University and co-author of the paper, told TODAY. “It’s now been confirmed by other groups using prospective approaches.”

They believe this information can help doctors counseling patients on the COVID-19 vaccine.

“It is incredibly important to make sure people are prepared and not surprised. If you went and got a vaccine and they didn’t tell you you’d have a fever the next day, it would be really concerning because you weren’t expecting it,” Lee said. “By knowing that changes to menstrual bleeding can happen, at least people are not surprised. They’re not shocked. They’re not worried that something major is wrong.”

A recent study showed that menstrual cycles changes from the COVID-19 vaccine are fleeting. Clancy believes that when providers share all the possible side effects of a vaccine, they can boost people’s “trust in the vaccines, in science.”

“There is a huge literature that shows that medical mistrust is one of the driving forces that makes people hesitant to get vaccines,” Clancy, associate professor of anthropology at University of Illinois Urbana-Champaign and co-author of the paper, told TODAY. “Making sure that people are not surprised or distressed and they know what to expect is going to bring them a lot of comfort.”

The results should be reassuring, experts said.

“The long term effects of a vaccine, in terms of changing, constricting cycle lengths or cycle flow was short lived,” Dr. Christine Mullin, OB-GYN and chief of Northwell Health Fertility Center in Manhasset, New York, who wasn’t involved in the paper, told TODAY. “Vaccines can potentially throw off our cycle a little bit. But most of the time, it’s just short lived and respond to it just like any other stressor.”

The findings also validate people who experienced menstrual changes but had been dismissed.

“Tons of people told us they had those experience, as well, where they told their doctor, their friend, their spouse, their partner and (they) were told that can’t happen,” Lee said.

Those experienced changes in menstruation

Lee and Clancy examined survey results from 39,129 adults who menstruate. They included women, transgender men, nonbinary people, people on long-acting birth control, postmenopausal people and those with disorders such as endometriosis who are often excluded from research. Of those with regular menstrual cycles, 42% reported heavier bleeding following vaccination.

“In terms of who was more likely to see this effect in our sample … people who were Hispanic were more likely to see heavier bleeding. People who were older in the pre-menopausal group were more likely to see heavier bleeding,” Lee said. “(People) diagnosed with ... something like endometriosis or fibroids were more likely to see heavy bleeding.”

Others impacted included: people who had been pregnant regardless of outcome and people who experienced other vaccine side effects, such as fever or exhaustion.

“If you’re generally having more of a reaction, we would expect to see (menstrual changes) also being increased,” Lee explained.

Postmenopausal people who experienced breakthrough bleeding tended to be younger. “That is probably because they have less time since they were menstruating,” Lee said.

Some trans people on hormones also experienced breakthrough bleeding, too.

“We have people who don’t normally menstruate — both people on long-acting, reversible contraceptive and people on gender-affirming hormones, like testosterone — as well as post-menopausal people, all of whom can experience breakthrough bleeding,” Lee said.

What causes changes in menstruation?

Clancy said the reason menstruation changes after the COVID-19 vaccine is that a lot of the uterus's "major activities are tightly tied to the immune system."

"One of the things that the uterus is constantly doing is ... building tissue. It’s differentiating tissue. It’s breaking it down. Menstruation is itself a process of tissue repair and tissue healing, much like wound healing you see anywhere else in the entire body," she explained.

When people receive a vaccine, it sparks immune responses throughout the body.

“If you have lots of immune activation happening, which always invokes that bleeding, clotting response in the body, then you imagine, wait is there an organ in the body that bleeds? So then you might think that the bleeding of that organ is going to be impacted,” Clancy said.

Mullin added that any type of stress can impact a person’s period, and for some, receiving a COVID-19 vaccine came with worries. Starting a new medication can often mean changes to one’s period, too.

“The COVID vaccine puts a stressor on people,” she said. “We typically tell our patients in starting birth control they may see irregularities in their menstrual (cycle) or what they call breakthrough bleeding. … Although it can be distressing, I don’t think it’s anything to worry about long term.”

Lee and Clancy plan to continue examining vaccines and menstruation, including a survey following up with some original survey participants.

“One thing we’re curious about is boosters, as well as ... how many periods does it take after the vaccine before it starts getting back to what your typical is?” Clancy said. “It’s really important to measure in the context of how does that change impact that person’s quality of life.”