Katharine Lee got her COVID-19 vaccination early on in the United States’ rollout. So did a friend of hers — they got their shots on the same day. They compared notes, curious to see what the side effects would be. Lee, a research fellow in the Division of Public Health Sciences at Washington University in St. Louis, says that they both had one surprising symptom: they both started their period soon after they took the vaccine.

“It wasn’t a symptom that was on the list,” she says. “I expected that my arm would be sore, or that I might have a fever or a headache, but this just wasn’t on the list.”

Lee reached out to Kate Clancy, who studies the menstrual cycle at the University of Illinois, to share the observation. When Clancy got her vaccination, she also had an unusual period. So she posted on Twitter asking if other people did, too — and watched dozens of responses roll in.

“It made us want to capture those experiences,” Lee says. So this week, the pair launched a formal study to collect data about the relationship between the COVID-19 vaccines and the menstrual cycle. It’s not a side effect that clinical trials checked for, and it’s not included on the Centers for Disease Control and Prevention’s vaccine health check-in.

There isn’t much research on how vaccination affects menstruation. But vaccines do stress the immune system, and the menstrual cycle sometimes responds to those types of changes. “The menstrual cycle is really dynamic, and it responds to tons of things,” Lee says.

There’s also a potential relationship between the nanoparticles used in the COVID-19 vaccines and changes in bleeding patterns. In some people, the nanoparticles end up creating a temporary immune reaction that kills off platelets, a type of blood cell involved in clotting, Clancy says. They regenerate quickly, but if someone has a bleeding event like a period just after they get a shot, it could make it heavier. She’ll be collaborating with Anirban Sen Gupta, a platelet researcher at Case Western Reserve University, to try to understand that potential mechanism.

If it turns out that there is a relationship between the vaccines and the timing of a person’s menstrual cycle, that doesn’t mean there’s something wrong or dangerous about the vaccines themselves. It would most likely be a temporary side effect, and probably wouldn’t have effects on fertility, Lee stressed. It wouldn’t be that different from other side effects that crop up with the vaccines, like a fever or headache. But it’s important to have a full understanding of what kinds of reactions the vaccinations can cause so that people are as informed as possible about what they might experience.

“It really sucks to be surprised by your period,” Lee says. “It’s nice to know that it could happen, in the same way that it is knowing that you might have a fever and headache.”

Similarly, unusually heavy periods are generally something people should be concerned about — they can be a sign that something is wrong, Clancy says. Knowing that it’s a possible effect of the vaccine could make the experience less concerning. On the other hand, if the research shows that there isn’t a strong link to vaccination, it may mean that people who do have this experience need to go to the doctor to make sure there isn’t something else wrong.

People are eager to share their stories on this survey, Clancy says. Within a day of launching the survey, there were already over 700 responses. They’re hoping to better understand how people’s periods are changing and the characteristics of people having those experiences — their age, gender, underlying health conditions, or other factors. They’re also tracking the timing. Clancy says she suspects that people who get the shot just before their period is supposed to start might be more likely to see heavy bleeding.

Right now, Lee says they’re hoping the survey fills a knowledge gap around menstruation and the vaccines. People have discussed their experiences on social media, but it hasn’t appeared in any scientific literature. Lee says she isn’t surprised. It took until the ‘90s for women to be regularly included in clinical trials of any kind, and some of those lingering biases remain.

“These are just not things some people think about. It’s not part of their daily lived experience. I think a lot of it is related to that history, and the bias around who gets to decide what’s important to consider as a side effect,” she says.