COVID-19 Vaccines Can Affect Menstrual Cycle, Researchers Find

A woman closes her eyes after receiving a COVID-19 vaccine at Sydney Road Family Medical Practice in Balgowlah, in Sydney, Australia, on Jan. 10, 2022. Jenny Evans/Getty Images

Researchers confirmed that COVID-19 vaccines are linked to changes in the menstrual cycle, according to a study published in March.

Published in the Obstetrics & Gynecology journal on March 1, Oregon Health & Science University researchers found that women who received a COVID-19 shot in the first half of their menstrual cycle are more likely to receive cycle length changes than those who received the vaccine in the second half.

Those researchers used data from 20,000 users of a birth control app that was approved by the U.S. Food and Drug Administration (FDA) to determine what effects the vaccine has on the cycle. Most of the women whose data was analyzed were under the age of 35, while 28 percent were from North America, 33 percent were from Europe, and another 32 percent were from the United Kingdom, they said.

Some were vaccinated and some were not. For those who were vaccinated, 63 percent received an mRNA vaccine, the paper said.

“Individuals vaccinated in the follicular phase experienced an average 1-day longer adjusted cycle length with a first or second dose of COVID-19 vaccine compared with their pre-vaccination average,” the authors of the paper said, referring to women who got a dose of the vaccine during the first half of their cycle.

Those who got the vaccine in the second half or those who were not vaccinated experienced no changes, they found.

The authors added that there is now “a body of evidence demonstrating that the ... vaccine is associated with temporary menstrual cycle disturbances at the population level,” adding that “the underlying mechanism for a vaccine-related cycle length disturbance is still under investigation.”

“The leading hypothesis is that these disturbances are due to the immune response that vaccines are designed to produce,” the study said, adding that “the immune and reproductive systems interact closely with one another.” Cytokines, which are small proteins that control the immune system’s activity and are produced “as an early event in the vaccine response,” can impact that process, they added.

Little research has been conducted in the past on how vaccines—whether for COVID-19 or others—could influence the menstrual cycle, the study’s authors further noted.

Responding to the study’s findings, Dr. Alison Edelman, the lead author of the paper with the Oregon university, said that “we do know the immune and reproductive systems interact closely with one another,” adding that with vaccinations, “it is certainly plausible that individuals may see temporary changes in their menstrual cycle due to the immune response.”

Their findings also suggested that there may be changes in the length of the cycle, although they appear to be short-lived. But they added that women who notice significant changes should contact a healthcare provider.

Previously, officials in Norway recommended women who experience heavy and persistent bleeding after vaccination put off any further doses until the cause is investigated or symptoms pass.

Other Research

And earlier in 2022, another set of researchers wrote that for women who received one of the COVID-19 vaccines, around 42 percent of respondents said they experienced increased menstrual bleeding. A majority of those who weren’t menstruating reported breakthrough bleeding after getting the shot, including two-thirds of women who were post-menopausal and slightly less than two-thirds of women who were using hormone treatments.

Most respondents received an mRNA vaccine made by either Moderna or Pfizer. But some also received Novavax, Johnson & Johnson, and AstraZeneca shots, according to the paper.

“We focused our analysis on those who regularly menstruate and those who do not currently menstruate but have in the past. The latter group included postmenopausal individuals and those on hormonal therapies that suppress menstruation, for whom bleeding is especially surprising,” Kathryn Clancy, a professor of anthropology at the University of Illinois Urbana-Champaign, said in a statement about the study’s findings at the time.

And it is not the first time that the same Oregon Health & Science University researchers found COVID-19 vaccines are associated with a change in the cycle. In 2022, they found that the change was pegged at under one day, and no change in menses length was detected.

While the study did not find vaccination associated with changes in menses length, “questions remain about other possible changes in menstrual cycles, such as menstrual symptoms, unscheduled bleeding, and changes in the quality and quantity of menstrual bleeding,” they wrote.

Pfizer Official’s Concerns

About a year ago, a Pfizer employee was seen in an undercover video telling a reporter with Project Veritas that he was concerned about the mRNA shot’s possible side-effects relating to menstrual cycles.

“There is something irregular about the menstrual cycles. So people will have to investigate that down the line because that is a little concerning,” the Pfizer official said in the video, adding that it “shouldn’t be interfering” with the cycles.

“I hope we don’t discover something really bad down the line,” he later added.

Zachary Stieber contributed to this report.