Yes, COVID vaccines can alter menstrual cycles, study says. What that really means

Since COVID-19 vaccines were created to guard against the infectious coronavirus, curiosity emerged about its effects on the body — including questions on how it could affect a person’s menstrual cycle.

A new study has sought to address concerns, particularly on social media, about if COVID-19 vaccination leads to “abnormal menstrual cycles.”

Researchers found that yes, vaccines can alter a period cycle — but here’s what that means in-depth.

It was discovered one dose of a COVID-19 vaccine is linked to “a small change in cycle length” as menstruating people who got the shot had slightly longer cycles — meaning their period was a little later than expected in the study published Jan. 5 in Obstetrics & Gynecology. The work received government funding from the National Institutes of Health.

Overall, the slight change was just under a one-day increase in the menstrual cycle length, “a longer time between bleeding,” the NIH said in a news release, and it didn’t affect the actual length of a person’s period, according to the study.

Researchers said small variation wasn’t “clinically significant” while defining a “normal cycle” length as 24-38 days.

It’s considered normal if a menstrual cycle’s length changes by less than eight days, according to the International Federation of Gynecology and Obstetrics, the study said. The timing of menstrual cycles can also be impacted by “life, environment, and health stressors.”

The work noted that 10% of individuals did experience a “clinically notable change in cycle length of 8 days or more,” but this change “quickly” lessened about two months after getting the COVID-19 vaccine — described as “two postvaccine cycles.”

Ultimately, the study suggests that the variations in menstrual cycles experienced after COVID-19 vaccination are temporary.

“These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly,” said Diana W. Bianchi, director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a statement.

Study authors did point out that given the lack of population-level and available evidence about the connection between COVID-19 vaccination and periods, the ability to “sufficiently” address concerns and give advice to individuals on menstruation amid vaccination is limited.

Menstrual cycle data of 3,959 people in the U.S. ages 18-45 was tracked through the app “Natural cycles,” the study said. Of those individuals, 2,403 were vaccinated and 1,556 unvaccinated individuals participated for comparison.

Of those vaccinated, 55% were vaccinated with Pfizer, 35% received the Moderna vaccine, and 7% got the J&J shot, according to the study.

“For an individual, small cycle changes can cause concern or raise hopes, especially if avoiding or planning for pregnancy, and this level of detail will likely be valuable,” study authors wrote.

Additionally, they acknowledged more research is needed to determine how COVID-19 vaccines “could potentially influence” other factors associated with periods such as “symptoms (pain, mood changes, etc.) and characteristics of bleeding (including heaviness of flow),” the news release said.

Certain study limitations mentioned included how the work “may not be generalizable to the U.S. population given the selection of Natural Cycles users (more likely to be White, college educated, and have lower BMIs than national distributions and not using hormonal contraception),” authors wrote.

Still, they said the “findings are reassuring” as “no population-level clinically meaningful change in menstrual cycle length associated with COVID19 vaccination” was found.

“Let’s say nobody told you that you were going to get a fever (after getting the Covid-19 vaccine). It’d be like, ‘What just happened?’” the study’s lead author, Dr. Alison Edelman of Oregon Health & Science University, told CNN.

“And people have different relationships with their menstrual cycle. For some people, maybe they’re planning pregnancy or trying to avoid a pregnancy. Even one day of change -- and that’s a mean -- can feel uncomfortable.”