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July 20, 2022

'I think it's important people know this can happen': Covid-19 vaccines linked to heavier periods

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    A new study published Friday in Science Advances found that 42% of people with regular menstrual cycles experienced heavier bleeding after receiving their Covid-19 vaccination, Knvul Sheikh reports for the New York Times.

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    Study details and key findings

    For the study, researchers at the University of Illinois at Urbana-Champaign and Washington University School of Medicine (WUSM) in April 2021 sent an online survey to thousands of people around the world.

    Three months after the survey was distributed, the researchers collected and analyzed over 39,000 responses from people between the ages of 18 and 80 about their menstrual cycles.

    All respondents were fully vaccinated with the Pfizer-BioNTech, Moderna, or Johnson & Johnson vaccines, or a different vaccine approved for use outside the United States. To the best of their knowledge, participants also had not contracted Covid-19 before they received the vaccine.

    According to the research, 42% of people with regular menstrual cycles experienced heavier bleeding after receiving the vaccine, 44% did not experience any change, and 14% said they had lighter periods. In addition, 39% of respondents on gender-affirming hormone treatments, 71% of respondents on long-acting contraceptives, and 66% of postmenopausal women reported breakthrough bleeding after one or both vaccine doses.

    Notably, the study also found that certain demographics may be more likely to experience menstrual changes. For example, older participants were more likely to experience heavier menstrual flow after vaccination.

    In addition, respondents who used hormonal contraception, had been pregnant in the past, or had been diagnosed with a reproductive condition—including endometriosis, fibroids, or polycystic ovarian syndrome—were more likely to experience heavier bleeding during menstruation. Researchers also found that many people who identified as Hispanic or Latino reported heavier bleeding.

    Respondents who experienced other side effects from the vaccines, such as fever or fatigue, had a higher chance of experiencing irregular periods.


    The study, which was the largest of its kind to date, expands on research that has explored the temporary effects of Covid-19 vaccines on menstrual cycles. However, previous studies primarily focused on cisgender women who menstruate.

    However, Katharine Lee, a biological anthropologist at WUSM and lead author on the study, warned that the study did not compare the results with a control group of people who did not receive the vaccine. While it is possible that people who experienced changes in their cycles following vaccination were more likely to participate in the survey, the findings support smaller studies that found menstrual changes following vaccination with more robust controls, Sheikh writes.

    While Covid-19 vaccines have effectively prevented deaths and severe illness with minimal side effects, many experts initially dismissed concerns when people who menstruate started reporting irregular menstrual cycles after vaccination.

    According to Keisha Ray, abioethics expert at McGovern Medical School at UTHealth Houston, public health officials should acknowledge concerns about menstrual cycle variations while warning people about the risk of getting Covid-19.

    "I think it's important that people know this can happen, so they're not scared, they're not shocked and they're not caught without supplies," said Lee.

    Ultimately, increased transparency around potential menstrual changes or any other side effects of the Covid-19 vaccine could help reduce people's vaccine hesitancy. In addition, Lee hopes that the new research will foster better conversations about people's health and result in more inclusive clinical trials in the future.

    "We're trying to be truthful. We're trying to validate people's lived experiences," Lee said. (Sheikh, New York Times, 7/15)

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