Kathryn Clancy got her first dose of a COVID-19 vaccine in early 2021 and 10 days later found herself sitting uncomfortably in a work Zoom meeting during one of the heaviest periods she’d experienced. “I had what’s often called menstrual flooding,” says Clancy, a biological anthropologist at the University of Illinois, Urbana-Champaign. Clancy wouldn’t have thought to connect the experience to the Moderna dose she’d received were it not for her graduate student, Katharine Lee, now at Tulane University, who shared a similar tale. “I had the worst cramps of my life” after COVID-19 vaccination, Lee says. Intrigued, Clancy—who didn’t experience similarly intense symptoms after her second dose—shared her story on Twitter. Hundreds responded with parallel stories, leading her to suspect a potential link to vaccination.
Those suspicions have grown with the completion of a more formal survey in which Clancy, Lee, and colleagues gathered thousands of stories of breakthrough bleeding and bleeding more heavily after a COVID-19 vaccine from people around the world. Between April and October 2021, the survey welcomed anyone 18 and older who gets or used to get a period. Because of that, Clancy cautions, the percentage affected is likely not representative of the general population—and indeed, a whopping 42% of about 16,000 people in the survey who had a regular menstrual cycle said they bled more heavily than usual after vaccination, a number far higher than others have reported. Still, the researchers captured a broad swath of people and their stories, including populations often neglected in menstrual research and breakthrough bleeding, which occurs when it is not expected, such as transgender and postmenopausal people.
Clarifying the issue is vital. “It’s important to know about,” says Victoria Male, a reproductive immunologist at Imperial College London. “Let’s say you got the vaccine and the next day you felt really dreadful the way some people do.” If you hadn’t been informed of the chance of fever, muscle aches, and other effects that quickly dissipate, “you would be really worried,” she said. Illuminating the chance of menstrual irregularities and confirming they aren’t a health risk also helps combat widespread misinformation that COVID-19 vaccines impair fertility, Male and others say.
“We need these studies out there because it elevates the issue,” says Alison Edelman, an obstetrician-gynecologist at Oregon Health & Science University who wasn’t involved in this research but has examined menstrual irregularities after COVID-19 vaccination.
After Clancy’s initial tweet drew such a response, she and Lee set out to learn more, initially aiming for a 500-person survey of menstrual experiences after COVID-19 vaccination. When 500 signed up in the first hour after the survey was posted online, Clancy realized the project would balloon well beyond what she’d anticipated.
More than 165,000 people around the world who were vaccinated with two doses participated. (Boosters weren’t widely available during the survey period.) Today the team reports in Science Advances on a subset of 39,000 from its first round of analyses. (The paper was posted in February as a preprint.)
Given their anthropology background, Clancy and Lee focused their survey on obtaining narrative-style responses and on documenting symptoms, such as breakthrough bleeding, that are important but can be hard to measure quantitatively. Among postmenopausal people, unexplained vaginal bleeding is a symptom of uterine cancer, so it happening after a vaccine dose could cause needless worry. In the survey results published today, 66% of 673 postmenopausal people reported breakthrough bleeding, as did 39% of the 280 people on gender-affirming hormones. Because of these hormones the latter group take, many don’t have a period and can find getting one distressing.
The survey results, even with the authors’ caveats, could lead to concern that the chance of menstrual irregularities after vaccination is much higher than it actually is, worries Lill Trogstad, an obstetrician-gynecologist and epidemiologist at the Norwegian Institute of Public Health who wasn’t involved with the study. In addition to likely attracting people who’d experienced these issues, the study didn’t include surveys of a control group of unvaccinated people or compare prevaccine menstrual cycles with postvaccine cycles in respondents over time. “These two things sort of limit the study,” Trogstad says. “You cannot estimate excess risk following vaccine” without a control group. Her own survey, done via smartphones and published as a preprint, found that just over 13% of young women in Norway said their periods were heavier after COVID-19 vaccination; that compared with 7% of the same group who said their period prior to vaccination was heavier than normal.
Clinical trials of COVID-19 vaccines didn’t look for effects on the menstrual cycle. Regardless of the prevalence of menstrual irregularities after COVID-19 vaccination, scientists note there are hypotheses that could explain it. Just like infections and fevers, which are known to affect periods, the immune system’s response to vaccination may alter sex hormone patterns or the cells that build up and break down the uterine lining, both of which are interlinked with immunology. There’s been limited research into other vaccines and periods, but one study from Japan found an association with the human papillomavirus vaccine and irregular menstruation.
The rash of anecdotes about menstrual irregularities after a COVID-19 vaccine, and resulting misinformation that the vaccines could disrupt fertility, spurred more studies on menstruation than the neglected topic usually receives. One March preprint by Male saw an effect on cycle length but not menstrual flow after vaccination in a small study of 79 people. A study this week on premenopausal women found a temporary half-day to 2-day increase, on average, in cycle length after a COVID-19 vaccine.
In her work, Edelman relied on anonymized data from a fertility awareness app. She found that, on average, vaccination altered cycle length by less than 1 day, but those who had both doses of vaccine in the same menstrual cycle experienced a 2-day difference, on average. About 10% of those individuals, who got both vaccine doses in the same cycle, saw a change of at least 8 days in their cycle length—but they appear to return to their baseline as soon as one cycle later. “We have this picture coming together that’s very reassuring,” Edelman says of these studies and others showing the vaccine doesn’t affect fertility.
For her part, Clancy is keen to continue with this research. She wants to study menstrual flow pre– and post–COVID-19 vaccination by having volunteers track and report symptoms in real time—rather than rely on memory, as this survey did. But she hasn’t been able to secure funding for such an effort. She’s also interested in studying the phenomenon after influenza vaccination. In the meantime, her team is wrapping up a follow-up survey on how quickly menstrual patterns renormalized for respondents. “Our goal is to start from the position of believing the people talking to us,” she says, and let that guide the studies to come.