Catherine Clancy received her Covid-19 vaccine in early 2021, and 10 days later found herself sitting comfortably in a booster meeting she was working on during one of the hardest times she’s ever had. “I had what is often called a period flood,” says Clancy, a biological anthropologist at the University of Illinois at Urbana-Champaign. If Clancy’s graduate student Kathryn Lee, now at Tulane University, had shared a similar tale, she wouldn’t have thought to connect her experience to Moderna’s size. “I had the worst cramps of my life,” says Lee, after the Covid-19 vaccine. Intrigued – Clancy, who didn’t experience the same symptoms after the second dose – shared her story. On Twitter. Hundreds responded with similar stories, leading her to suspect it might be related to the vaccine.

Those doubts grew as Clancy, Lee and their colleagues completed a more formal survey in which they collected thousands of bleeding and bleeding stories from people around the world after the COVID-19 vaccine. Between April and October 2021, the survey is open to anyone aged 18 and over who experiences or is used to menstruating. Because of this, Clancy cautions, the affected percentage is not representative of the general population—and indeed, 42% of the 16,000 people in the study with normal menstrual cycles experienced heavier bleeding than usual after the vaccine. The number is much higher than other reports. Still, the researchers caught up with many people and their stories, including those who have been overlooked in research on menstruation and when bleeding is not expected, such as transgender and postmenopausal women.

It is important to clarify the issue. “It’s really important to know,” says Victoria Male, a reproductive pathologist at Imperial College London. Let’s say you get the vaccine and the next day you feel terrible, as some people do. If they hadn’t told you about the fever, muscle aches and other effects that go away quickly, “you’d be very worried,” she said. Shedding light on the possibility of menstrual irregularities and making sure they’re not a health risk could help fight misinformation about how Covid-19 vaccines can harm fertility, say Male and others.

“We need these studies out there because they raise the issue,” said Alison Edelman, an obstetrician-gynecologist at Oregon Health & Science University who was not involved in the study but has examined menstrual irregularities after the Covid-19 vaccine.

After Clancy’s first tweet drew such a response, she and Lee set out to learn more by planning a survey of 500 people about menstrual experiences after the Covid-19 vaccine. When 500 people signed up within the first hour of posting the survey online, Clancy realized the project was going to be bigger than she imagined.

More than 165,000 people worldwide were vaccinated with two doses. (Incentives were not widely available during the survey.) Today, the team reports Scientific developments A subset of 39,000 from the first round of analysis. (The paper was published in February As a pre-print.)

With an anthropological background, Clancy and Lee focused their survey on eliciting narrative-type responses and documenting symptoms such as bleeding, which are important but difficult to quantify. Among postmenopausal women, unexplained vaginal bleeding is a symptom of cervical cancer, so it can cause unnecessary stress after a dose. In a survey published today, 66 percent of 673 postmenopausal women reported bleeding problems, and 39 percent of 280 women who received gender-affirming hormones. Because of these hormones that the latter group takes, many do not have a period and may find it bothersome.

The survey results, despite the authors’ warnings, may raise concerns that the incidence of post-vaccination menstrual irregularities is far higher than it actually is, worries Lille Trogstad, an obstetrician and epidemiologist at the Norwegian Institute of Public Health. He was not involved with the study. In addition to recruiting people who experienced these problems, the study did not include surveys of a control group of unvaccinated people or compare pre-vaccination menstrual cycles with post-vaccination cycles of respondents over time. “These two things limit the study,” says Trogstad. Without a control group, “you can’t estimate excess risk after vaccination,” her own study, conducted with smartphones and published as a preprint, found that More than 13% of young women In Norway, they said their periods were heavy after the Covid-19 vaccine. Compared to 7% of the same group who said their periods were heavier than usual before the vaccine.

Clinical trials of Covid-19 vaccines have not found an effect on the menstrual cycle. Regardless of the cause of menstrual irregularities after the Covid-19 vaccine, scientists note that there are hypotheses that could explain it. Just as infections and fevers affect menstruation, the immune system’s response to vaccines can alter sex hormone patterns or the cells that build and break down the lining of the uterus, both of which are linked to immunology. There is limited research on other vaccines and periods, but one study from Japan A link has been found with the human papillomavirus vaccine and irregular periods.

A rash of anecdotes about menstrual irregularities after the covid-19 vaccine and its consequences Misinformation that vaccines can damage fertilityThe neglected topic of menstruation has inspired more research than it often receives. A March pre-print by the male saw It affects the length of the cycle, not the period. After vaccination in a small study on 79 people. This week’s study on premenopausal women a Temporary half-day increase to 2-dayOn average, by cycle length after the Covid-19 vaccine.

In her work, Edelman relied on anonymous data from the Fertility Insights app. She is, on average, The vaccine is changed with a cycle length of less than 1 day, but those who received both doses of the vaccine in the same menstrual cycle experienced an average 2-day difference. About 10% of people who took both doses of the vaccine in the same cycle had their cycle length change for at least 8 days – but seemed to return to baseline after one cycle. “It’s very comforting to see this picture come together,” Edelman said of these studies and others. The vaccine does not affect fertility.

For her part, Clancy is eager to continue this research. Instead of relying on recall, as this survey did, she wants to study menstrual flow pre- and post-Covid-19 vaccination by having volunteers monitor and report symptoms in real time. But she was unable to secure funding for such an endeavor. She is also interested in studying what happens after influenza vaccination. In the meantime, her team is completing a follow-up survey of how quickly menstruation resumes among respondents. “Our goal is to start from a place where we believe the people we’re talking to,” she says, and let that guide future research.

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