When women started reporting longer periods and heavier bleeding than normal after receiving Covid vaccines last year, there was little data to back it up.
Although they made up about half of the participants in the Covid vaccine trials, women were not asked about any menstrual changes as part of this process. Since then, several studies have found that Covid vaccines can indeed induce short-term changes in menstrual cycles.
So a growing chorus of researchers is calling for more study of the effects of vaccines on menstruation. Collecting this kind of data during Covid vaccine trials, they say, could have prevented the distress of those who experienced abnormal changes in their cycles and allayed fears about vaccines at a time when misinformation abounded.
“Because we didn’t have data and people weren’t paying attention to it, people who started reporting it were blown away. People feel like they’re in the spotlight and have those concerns,” said Alison Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University.
In an op-ed published Thursday in the journal Science, Dr Victoria Male called for future trials to ask people about period changes and to take respondents seriously if they report such side effects.
“We shouldn’t underestimate how important it is to do this – to listen to people and address their concerns, especially around public health interventions like vaccines,” said Male, senior lecturer in Reproductive Immunology at Imperial College London.
If a side effect is unstudied and unexplained, she added, it can fuel mistrust and misinformation; in this case, the false narrative that menstrual changes from Covid vaccines may have been linked to fertility issues.
“There’s been a lot of very simple misinformation about how vaccines might impact fertility, and we have so much evidence that it doesn’t, but you can understand how people might link the dots here and worry,” she said.
Evidence so far suggests Covid vaccines can temporarily prolong menstruation or lead to heavier periods, but these vaccine-induced changes are short-lived – Male said most were resolved in one or two rounds.
A study published in January by the Norwegian Institute of Public Health identified a significant increase in heavier than normal bleeding in women aged 18 to 30 after their Covid shots. And in a July analysis, 42% of those with regular menstrual cycles reported bleeding more heavily than usual after vaccination.
Edelman’s research earlier this year showed that the first and second doses of the Covid vaccine were each associated with longer than normal menstrual cycles – less than an extra day of menstruation – in the United States. His study found no such association in the unvaccinated.
A follow-up study showed the results to be true for people in Canada and Europe.
“When you see something like this happen, then you’re like, ‘Wait a second, why didn’t we have this data before? ‘” Edelman said.
Male said the oversight could be due, in part, to the fact that most vaccines are given during childhood, so menstruation is not a priority in trials. But the problem reflects greater inequality in clinical research, she added.
“It wasn’t until the early ’90s that it was mandatory to always include female participants in your research,” she said. “Before this requirement existed, a lot of people would deliberately do everything about male participants because, in some ways, it’s easier. The ramifications of that continue to echo.”
A 2021 study found that only eight of 45 clinical trials that tested Covid vaccines and therapies separated results by gender. Another study found that eight of 121 Covid-related trials analyzed whether gender affected their main results.
In the past, menstrual changes have also been reported in people vaccinated against typhoid, hepatitis B and influenza. Male said the largest study documenting such changes found that teenage girls in Japan who received the HPV vaccine were more likely to go to hospital for heavy or irregular periods.
“The clues were there for us for a while if anyone thought to connect the dots, but I guess no one did,” she said.
Edelman said these reports have sometimes been dismissed as part of the normal variation in menstrual cycles.
But it wouldn’t be difficult to collect data on menstruation in clinical trials, according to Sabra Klein, co-director of the Center for Women’s Health, Sex and Gender Research at the Johns Hopkins Bloomberg School of Public Health.
“It would be a question of adding a question which may include sub-questions: do you have your period? And if the answer is yes, have you experienced a change in your menstrual cycle? she says.
Instead, Klein said, menstrual side effects first came to light because women spoke out on social media. Over time, problems with menstruation began to be reported to the federal vaccine adverse event reporting system. By April, more than 11,000 had filed such reports.
In addition to longer or heavier menstrual cycles, women have also reported shorter or lighter cycles than normal, as well as missed periods, but these other changes have yet to be confirmed by studies.
Researchers don’t know why post-vaccination menstrual changes occur. The main theory is that vaccines trigger an immune response that interacts with hormones, which then signal the body to delay ovulation and shed the uterine lining for longer.
Timing of vaccination may also matter: a July study, which was not peer-reviewed, found that participants saw slightly larger increases in cycle length when they were vaccinated during the first half of their cycle than during the second.
Edelman said more data on this could allow people to make more informed decisions about when to plan future shots.
“If you’re going camping or on a trip or you’re getting married and you’re thinking, ‘Wouldn’t it be a good idea to have my periods different this month’, you might decide to take [vaccinated] a different month,” she said.