A new research published in the journal Science Advances, adds to the evidence that after COVID-19 vaccination, significant numbers of people experience this unexpected side effect.

A new review of more than 35,000 people provides the most thorough assessment so far of menstruation changes in the first two weeks after receiving the COVID-19 vaccine.

The findings of the study, which were published in the journal Science Advances, contribute to the growing body of evidence indicating that a large population is affected by this unfortunate side effect.

The researchers who oversaw the study noted that starting in early 2021, “menstruating and formerly menstruating people began sharing that they experienced unexpected bleeding after being administered a COVID-19 vaccine in early 2021.” This side effect was frequently neglected or dismissed because vaccine trials don’t ask about menstrual cycles or bleeding.

According to Kathryn Clancy, doctors who weighed in on the matter after hearing early accounts regarding post-vaccination menstruation changes were often dismissive of patients’ worries. According to some medical professionals, it’s unclear how a vaccine might cause such changes.

Clancy said that other vaccines, such as those for typhoid, Hepatitis B, and HPV, can sometimes cause changes in menstruation.

These adverse effects are less likely to be caused by hormonal changes and are thought to be linked to an increase in immune-related inflammatory pathways.

“We suspect that for most people the changes associated with COVID-19 vaccination are short-term, and we encourage anyone who is worried to contact their doctor for further care,” Lee added. 

A survey was used to ask people about their experiences after getting a shot. The survey, which was introduced in April 2021, focused on respondents’ reproductive histories and experiences with menstrual bleeding while also collecting demographic and other information.

On June 29, 2021, the team downloaded the survey data. The investigation only included individuals who had not yet received a COVID-19 diagnosis because this condition has been linked to menstrual changes in the past.

The study also left out data from people 45 to 55 years old so that menstrual changes related to perimenopause wouldn’t mess up the results.

“We focused our analysis on those who regularly menstruate and those who do not currently menstruate but have in the past,” the author said. “The latter group included postmenopausal individuals and those on hormonal therapies that suppress menstruation, for whom bleeding is especially surprising.”

Statistical analysis showed that 42.1 percent of women who had their periods after getting the COVID-19 vaccine had heavier periods.

Some people reported this during the first week, but many others noticed improvements eight to fourteen days after the vaccine. According to the researchers, around the same percentage, 43.6 percent, reported no change in their menstrual flow following the vaccine, and a smaller percentage, 14.3 percent, noticed a mix of no change and lighter flow.

Lee said that because the study was based on self-reported experiences that happened more than 14 days after vaccination, it can’t prove a cause-and-effect relationship or be used to make predictions about the general population.

However, it may indicate connections between menstrual changes after COVID-19 vaccination and a person’s reproductive history, hormonal state, demography, and other factors.

For example, the analysis showed that people who had given birth were more likely to say they had heavier bleeding after getting vaccinated, while people who had never given birth only had a slight increase.

Most non-menstruating premenopausal respondents on hormone treatment had breakthrough bleeding.

Over 70% of respondents using long-acting reversible contraception had this side effect, whereas 38.5% of respondents getting gender-affirming hormone therapy also reported this.

Participants were more likely to report larger menstrual flow following vaccination if they identified as non-white, Hispanic/Latinx, older, or any of these categories, as well as if they reported fever or exhaustion as a side effect of the immunization.

Those with a history of reproductive health issues, such as endometriosis, menorrhagia, fibroids, or other conditions, were also more likely to report an increase in menstrual flow after vaccination.

Lee said that unexpected changes in menstruation can still cause worry, even if the increase in flow is temporary and goes away quickly.

“Unexpected breakthrough bleeding is one of the early signs of some cancers in post-menopausal people and in those who use gender-affirming hormones, so experiencing it can make people worry and require expensive and invasive cancer-screening procedures,” Lee added.

“This screening is very important so we can catch cancers early,” Clancy added. “Anyone with breakthrough bleeding should see their doctor. For diagnostic purposes, it would be helpful to know whether there are other causes for the bleeding.”

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