Experienced erratic menstruation after Covid shot? New study shows how vaccines affect period cycles


Nearly half of the participants in a recent study who were menstruating regularly at the time of the survey reported more heavy bleeding during their periods after receiving the COVID-19 vaccine. Others who did not normally menstruate, including transgender men, people taking long-acting birth control, and postmenopausal women, also experienced unusual bleeding.

The new study – the largest to date – expands on studies that have highlighted the temporary effects of COVID-19 vaccines on menstrual cycles, but so far has focused on cisgender women who are menstruating.

Although the vaccines largely prevented deaths and serious illnesses with few reported side effects, many medical experts initially brushed aside the concerns when women and people of different sexes began to report irregular menstrual cycles after receiving the vaccines.


To better understand this post-vaccination experience, researchers at the University of Illinois at Urbana-Champaign and the Washington University School of Medicine in St. Louis distributed an online survey to thousands of people around the world in April 2021. After three months, the researchers collected and analyzed more than 39,000 responses from people aged 18 to 80 about their menstrual cycles. All survey respondents were fully vaccinated with Pfizer-BioNTech, Moderna, Johnson & Johnson, or other vaccines approved outside the United States. And, to the best of their knowledge, participants did not contract COVID-19 prior to vaccination.

A study published Friday in the journal Science Advances shows that 42% of people with regular periods experienced more bleeding after vaccination, while 44% had no change and 14% had lighter periods. In addition, 39% of respondents receiving sex-confirming hormone therapy, 71% of people taking long-acting contraceptives, and 66% of postmenopausal women experienced breakthrough bleeding after one or both injections.

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“I think it’s important that people know that this can happen so that they don’t get scared, shocked, or run out of supplies,” said Katherine Lee, a biological anthropologist at the University of Washington School of Medicine in the US. St. Louis and first author of the study.

Lee cautioned, however, that the study did not compare the results with a control group of people who had not been vaccinated. And it is possible that people who observed changes in their cycles after vaccination were more likely to participate in the survey. However, the results are consistent with smaller studies reporting menstrual cycle changes after vaccination with more robust controls.

Importantly, the new study also found that certain demographics may be more likely to experience menstrual changes, and the study could help them prepare better, Lee said. For example, heavier menses were more likely to occur in older people. Survey respondents who used hormonal contraception, were pregnant in the past, or who were diagnosed with reproductive disorders such as endometriosis, fibroids, or polycystic ovary syndrome, were also more likely to have more heavy bleeding during menstruation. People who self-identified as Hispanic or Latino also tended to report more heavy bleeding. And people who experienced other side effects of vaccines, such as fever or fatigue, also had a higher chance of experiencing intermittent periods.

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Postmenopausal women who were slightly younger, averaging around 60 years of age, were more likely to experience breakthrough bleeding after the vaccine than those who were older. But the type of vaccine the postmenopausal women received, whether they had other side effects such as fever, or whether they had a previous pregnancy, did not appear to affect their bleeding.

Why are these changes happening?

Some level of menstrual variability — the number of days of bleeding, the amount of bleeding, and the length of the cycle — is normal.

“Our menstrual cycles are not perfect hours,” said Dr. Alison Edelman, professor of obstetrics and gynecology at


Health & Science University, which also studied the effects of COVID-19 vaccines on menstruation.

Hormones secreted by the hypothalamus, pituitary gland, and ovaries regulate the menstrual cycle and can be influenced by both internal and external factors. Stress and illness, weight loss or gain, calorie restriction, and intense exercise can change the typical pattern of menstruation.



Experts agree that the havoc that COVID-19 can wreak on your body, including potential lingering effects, is far worse than any side effects caused by getting vaccinated against the disease.

The endometrium, which lines the uterus and is released during menstruation, is also associated with the immune system. Because of the role it plays in remodeling uterine tissue and providing protection against pathogens, it is possible that when vaccines activate the immune system, which they are supposed to do, they also somehow cause downstream effects in the endometrium, causing disruption. in your menstrual cycle, Edelman said. And some people may be more sensitive to immune or hormonal changes in their bodies.

In her study, Edelman found that some women menstruated a day or two later than usual after they received the coronavirus vaccine. But the changes were temporary – menstruation, as a rule, returned to normal after one or two cycles.

What to do if you notice menstrual irregularities after a COVID shot

If you find any new or unusual bleeding patterns, look into it. The menstrual cycle can be seen as another vital sign, like body temperature or blood pressure, that gives an indication of your health, says Dr. Jennifer Cavasse, a reproductive endocrinologist at Emory University, who was not involved in the study.

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“A significant change in menstrual interval or bleeding profile warrants further investigation to ensure there is no underlying endocrinological, hematological, or anatomical cause,” Cavasse said. For example, breakthrough bleeding in people who normally no longer have periods can also be a warning sign of cervical, ovarian, uterine, or vaginal cancer.

That being said, small changes in your menstrual cycle if you have regular periods shouldn’t be a cause for concern and don’t require you to change anything you normally do, Cavasse said.

Clinical trials and other studies have already established that COVID-19 vaccines are safe and effective and are unlikely to affect fertility in the long term.

Do I need to be vaccinated at a specific cycle time?

Experts agree that the havoc that COVID-19 can wreak on your body, including potential lingering effects, is far worse than any side effects caused by getting vaccinated against the disease.

People who have previously had a fever after a shot can schedule their next dose for a day when they don’t have to go to work, Edelman said. But you shouldn’t let temporary menstrual changes prevent you from getting a full vaccination or boost. As the number of cases rises again, delaying vaccination by two weeks or longer could significantly increase the risk of contracting COVID-19, she said.

However, it’s important to monitor your body’s response to vaccinations, and public health officials should acknowledge concerns about menstrual fluctuations in addition to warning people about the risk of contracting COVID-19, said Keisha Ray, a bioethicist at UTHealth’s McGovern School of Medicine. Houston.

Increasing transparency about menstrual changes or other side effects of vaccination may also have another benefit: reduce people’s hesitation about getting vaccinated.

“We are trying to be truthful. We’re trying to validate people’s life experiences,” Li said. In turn, she hopes the new study will help improve conversations about people’s health and lead to more inclusive clinical trials in the future.


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