New Study Shows COVID Vaccine Temporarily Increases Menstrual Cycle By 1 Day

For some, receiving a COVID-19 vaccine meant muscle soreness, fever or exhaustion — all side effects that experts warned people could occur. Others noticed an unexpected reaction: a change in their menstrual cycle. A new study published in the journal BMJ Medicine finds that receiving a COVID-19 vaccine causes a slight increase in the length of one cycle, reinforcing the findings of a previous paper that looked at data from U.S. participants.

“We found a small change in cycle length of about a day in individuals that got one vaccine in a cycle as compared to an unvaccinated group. And then for individuals that received two vaccines in a cycle — if you recall the initial vaccines were supposed to get about a month apart — those individuals experienced a greater cycle change so almost up to four days,” Dr. Alison Edelman, an author of the study and professor of obstetrics and gynecology and division director of Complex Family Planning in the Oregon Health & Science University School of Medicine, told TODAY.

“What’s very reassuring is it looks like the cycle length starts going back to normal post-vaccination.”

This means that if a person received one vaccine dose during their cycle, which might normally be 28 days, it could be 29 days long instead. If they received two shots during one cycle, then the time between their periods could be as long as 32 days.

For this study, Edelman and her colleagues looked at data from 20,000 people using an app, Natural Cycles, to track their periods. Some people received the vaccine while others, the control group, did not. Researchers looked at three cycles before the vaccine and then cycles during vaccination and after to understand the participants’ schedules.

“We really needed to see pre-vaccination cycles immediately before the vaccine to see what their baseline is to see if a changed happened,” Edelman said. “Everybody’s cycle is somewhat changeable, some people more than others, and so we have to see is that change related to the vaccine or not.”

These finding reinforce what Edelman and her colleagues found when looking at U.S. data. For many, this should be validating and reassuring.

“It’s causing a slight disturbance,” Dr. Adi Katz, director of gynecology at Lenox Hill Hospital in New York City, who was not involved in the research, told TODAY. “The COVID-19 vaccine is similar to the COVID infection in that it causes an immune response in the body and a lot of the process of menstruation is also immune mediated.”

This information can help doctors counsel patients about vaccination and include changes in menstruation as a side effect of the vaccine — much like fever or body aches.

“The research so far has shown that it’s a very short lived abnormality in the menstrual period. I can tell you from my clinical practice that for almost all the patients that it resolved within the first two cycles after the booster and the vaccine. And we haven’t seen any long term effects,” Katz said. “If you continue to have irregular periods you should go see a doctor. It may have nothing to do with the vaccine.”

Katz adds that people worried that the COVID-19 vaccine impacted fertility and she stressed that this is not true.

“The studies have shown that there’s no long term effect on fertility or getting pregnant,” she said. “It’s important to know that.”

This study bolsters anecdotal reports from people who noticed changes in their periods when first undergoing vaccination for COVID-19.

“If we look back at the beginning of vaccination and the pandemic and the concerns that people had and then what people were reporting, we didn’t have the information to tell them if this was something that was happening or not,” Edelman said. “Now we have a number of studies either retrospectively and then this prospective data that helps solidify the reports.”

The study can also contribute to a better understanding of menstruation.

“We’ve always known there are some external factors that change menstrual cycles and we know just a serious infection or when people get fevers that their menstrual cycle fluctuates,” Edelman said. “What this data does not tell us is perhaps why there are temporary changes. It’s likely because of immune cross talk with our reproductive system.”

Edelman said she hopes that studies of new vaccines in the future will examine their impact on menstruation.

“There is a pretty easy way to do that for individuals that are doing vaccine studies and I hope that they’ll incorporate it because it allows the public information around an outcome, menstrual cycles, that they’re really interested in,” she said. “By providing that information it gives people reassurances around vaccines and an understanding of what to expect.”

This article was originally published on TODAY.com