Women say COVID vaccines affected their periods. A new study adds to mounting evidence.

I've spent the majority of my adult life on birth control pills, taking them continuously, so my period has been nonexistent. So imagine my surprise when, in December 2021, I got my period, along with excruciating menstrual cramps. It was so out of the norm, I went to the emergency room, concerned that the pain and blood was a sign of something scary like an ectopic pregnancy or an undiagnosed disease.

I never got an official answer in the emergency room or with the gynecologist I followed up with shortly after my ER visit. It wasn’t until I sought out a second gynecologist a few months later did she suggest my symptoms may have something to do with my recent COVID vaccine.

While I had also experienced the more typical post-shot reactions of slight fever and muscle aches, it never occurred to me that my COVID vaccine might have an impact on my menstrual cycle. But there is mounting evidence to support the theory that it can.

The latest, a September 2023 study published in Science Advances, by Kristine Blix at the Norwegian Institute of Public Health in Oslo, found that women who don’t menstruate were several times more likely to experience unexpected vaginal bleeding after COVID-19 vaccination than before the vaccines were offered. This includes postmenopausal women as well as people like me, who are pre-menopausal but regularly skipped their period due to contraceptive use.

Before that, in January 2022, a National Institute of Health-funded study found that a COVID-19 vaccination was associated with a small, temporary increase in menstrual cycle length.

And a year ago, in September 2022, a large study published in BMJ Medicine, which looked at data from nearly 20,000 people, confirmed the link between COVID-19 vaccination and temporary increase in menstrual cycle length.

Outside of these published studies, there’s also quite a bit of anecdotal evidence — at least, that’s what I found when I scoured Reddit, trying to find an answer for my symptoms.

“After my shot I basically had 2 periods a month,” one person wrote in a December 2022 thread. “It was like I’d have a period way heavier than they were before my vaccine.”

Another said, “After my mom got the shot she got her period. She had already gone through menopause eight years ago and never even spotted throughout those years. As soon as she got vaccinated she started bleeding the next day for a week. Never got it again after that. It was so weird and she went through so many tests and everything was normal.”

A third wrote that they “spotted for the entire month of July” and that their periods became “so severe” that the cramps made them vomit, which unfortunately sounded similar to my experience.

Kate Clancy, a biological anthropologist at the University of Illinois, also noticed a flood of anecdotal evidence, as she wrote in a recent op-ed for the Washington Post — ultimately leading her and a colleague to conduct a 165,000-person survey of menstruation patterns post-vaccination.

“Our survey was intended to document participants’ experiences with the early vaccine rollout — to collect these data on the ground, as they emerged — and, as such, our sample was self-selected. But the sheer volume of respondents surprised by heavy and breakthrough bleeding was striking,” she wrote. “Changes in menstruation were uncomfortable and often painful, people reported. Strangers described bleeding through their clothes at work. Postmenopausal people were terrified they had cancer.”

So what’s going on here?

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, says it’s important for people to know the response makes sense. While the theory has to be studied further, COVID vaccines cause an inflammatory response in the body, which is why some people get a fever or chills hours after your shot. Other things, such as illness, create an inflammatory response, which can affect a menstrual cycle.

"In general, we've known for some time that inflammation does influence menstrual cycles, including the amount of bleeding and all of the variables associated with menstruation that can be impacted by an immune response," he explains.

It's apparently only come to widespread attention now, Adalja suggests, because as a society we've been “very closely tracking COVID-19 vaccines” in particular.

But, as noted in a New York Times opinion piece by Alice Lu-Culligan and Dr. Randi Hutter Epstein, both of the Yale School of Medicine, clinical trials do not typically track and document menstrual changes as with other responses, which, they argue is “part of a long history of medicine not taking women’s bodies seriously.”

Such menstrual changes make sense, although "since the cycle is supported by the immune system at every turn, it is possible that the vaccines, which are designed to ignite an immune response, temporarily change the normal course of events. For example, an activated immune system might interfere with the usual balance of immune cells and molecules in the uterus. These types of disturbances have been found in studies to contribute to changes in periods, including heavy menstrual flows."

Changes like these are a potential, temporary side effect of the vaccine, which should be discussed with your doctor.

It's worth noting that “it’s not something that is associated with any kind of health risk," Adalja says. However, in a postmenopausal person, “that often prompts a lot of investigation to rule out things like endometrial cancer. It’s important for doctors, when evaluating postmenopausal bleeding, that the vaccine may be responsible for it and not necessarily an endometrial tumor.”

As someone who was concerned about what was going on in my body following my shots, this would have been a key piece of information to have.

Dr. Elizabeth Swenson, an ob-gyn at Wisp, agrees: “It’s always important to conduct research into how and why medications impact people’s bodies. ... The menstrual cycle is a key component of women’s health, and knowing as much as we can about anything that can potentially impact it is imperative to the future of women’s health.”