When adults gained access to Covid vaccines last year, most knew to expect headaches, fatigue and soreness as side effects.
But some researchers think it’s time to add another common one to the list: temporary menstrual changes.
An analysis published Friday in the journal Science Advances found that 42% of people with regular menstrual cycles said they bled more heavily than usual after vaccination. Meanwhile, 44% reported no change and around 14% reported a lighter period. Among nonmenstruating people — those post-menopause or who use certain long-term contraceptives, for example — the study suggests many experienced breakthrough or unexpected bleeding after their Covid shots.
The survey included over 39,000 people 18 to 80 years old who were fully vaccinated and had not contracted Covid. The study authors cautioned, though, that the percentages do not necessarily represent the rate of menstrual changes in the general population, since people who observed a difference were more likely to participate. The survey’s aim was simply to provide evidence for future studies, not to establish cause and effect.
Still, other recent research also found that the Covid vaccine is associated with a small change in menstrual cycle length.
The new survey started in April 2021, around the time people began to report unexpected bleeding and heavier flow post-vaccine. However, these anecdotes were at the time met with the rebuttal that there was no data linking menstrual changes to vaccination.
That was both true and indicative of a larger problem. Individuals who took part in Covid vaccine trials were not asked if they experienced menstrual changes.
“Before the vaccinations came out, I would say our knowledge on the subject of the connection between immunization and menstrual changes, in general, was nil,” said Candace Tingen, a program director with the gynecologic health and disease branch of the National Institute of Child Health and Human Development. Tingen was not involved in the recent survey.
Overall, few studies assess the direct effect of vaccination on the menstrual cycle, and most pharmaceutical trials have not included questions about changes to menstruation.
Tingen views this as a mistake. Perhaps, she said, if Covid-19 vaccine trials had asked about menstruation, people would not have been surprised — or frightened — by this unexpected side effect.
“It was really this lack of information that I think caused confusion, fear and perhaps vaccine hesitancy,” she said.
Study co-author Katherine M.N. Lee said that overall, menstruation is understudied when it’s not relevant to pregnancy.
“It gets ignored because of the structure of science,” Lee, an assistant professor at Tulane University, said. “There are very few senior people in science and medicine who are not white men. It’s just not something they are thinking about as part of their lived experience.”
Lee and her colleagues were inspired to ask people about their menstruation cycles after being vaccinated after seeing both friends and strangers online wonder why they experienced an unexpected change.
The survey group included more than 3,500 people who identify as gender diverse. Approximately 84% of participants were white, and none were between the ages of 45 and 55 because the researchers didn’t want to include changes associated with perimenopause, when the body begins the transition to menopause.
The respondents were vaccinated with Pfizer, Moderna, AstraZeneca, Johnson & Johnson and Novavax.
The group included people who do not usually menstruate because they are post-menopausal, use long-acting reversible contraceptives or hormonal contraceptives, or are undergoing gender-affirming treatment that stops menstruation. A majority of these respondents experienced breakthrough bleeding after the vaccine.
Among the 238 postmenopausal people in the study who were not on hormonal treatments and had not bled for at least 12 months before their vaccination, 66% reported breakthrough bleeding.
The survey found that in general, people who experienced a heavier flow after their shots were more likely to be nonwhite and older; to use hormonal contraception; to have a diagnosed reproductive condition; to have also experienced fever or fatigue as side effects; or to have been pregnant in the past.
As part of the survey, the team also included free response sections where participants could share their experiences.
“A large number of people reported the feeling that ‘I’m so angry that I didn’t know this ahead of time, but I’m glad I still got it,’” said Kathryn Clancy, an associate professor of anthropology at the University of Illinois and the study's other co-author. “They wouldn’t have changed their decision to get the vaccine, but they felt betrayed by the fact that no one told them to expect it.”
It’s not yet understood why menstrual changes happen after vaccination. Tingen said the answer will likely stem from the overlap between the immune system and endocrine system, which plays a role in reproduction. (There are immune cells in the uterus that help during the process of menstruation, for example.)
“Diet, sleep and stress can all affect the menstrual cycle,” Tingen said. “It is perhaps not surprising that a large immune reaction might throw menstruation off temporarily, in a minor way.”
Covid itself has been shown to disrupt menstruation more significantly than vaccines. This may be because of an ongoing immune reaction and the lifestyle changes that come with being sick. People with long Covid are especially affected, Tingen said, and more research on the reason is needed.
Menstrual changes as a vaccine side effect are on par with others like muscle pain: While uncomfortable, they do not change how safe or effective the shots are. But Clancy said unexplained side effects cause people to feel fearful — and in some cases spread vaccine hesitancy.
Some people skeptical of vaccines have falsely conflated the possibility of short-term menstrual changes with long-term harm to fertility, messaging that Lee described as an “active disinformation campaign.”
Clancy said she’s received messages from parents who’ve heard about menstruation changes and are concerned that vaccinating their child will cause early puberty, even though there is no evidence for this.
Period-related surprises post-vaccine posed particular challenges for trans men and genderqueer people, according to the study, since some had to navigate public or workplace bathrooms after experiencing unexpected menstruation.
“Unexpected bleeding runs the risk of psychological distress for those who experience gender dysphoria with menstruation and physical harm for people for whom managing menstruation in public is dangerous,” the authors wrote.
Clancy, Lee and their colleagues hope their work inspires further research, encourages clinicians to talk to patients about the link between vaccination and menstrual changes, and validates people who have felt ignored or alone in their experiences.
“If you want to improve trust in government, trust in pharmaceutical companies, trust in medicine, trust in vaccines, then you have to take the time to do the work, so people know what to expect going into it,” Clancy said. “That effort makes people more likely to get their second shot or booster.”