New evidence confirmed that among the side effects people experience after getting the COVID-19 vaccine were lesser-known issues, such as heavier periods and breakthrough bleeding.

As a normal sign that the body is developing protection, the COVID vaccine leaves some side effects, including fever, tiredness, headache, muscle ache, and chills.

However, a new study published in the journal Science Advances reported that people with endometriosis, heavy bleeding, fibroids, or other types of reproductive problems were more likely to experience a heavier menstrual flow after receiving a vaccine shot.

The researchers behind the study analyzed responses from over 35,000 people within two weeks after receiving the vaccine. They found that over 42% of menstruating survey respondents reported a heavier flow after getting the shot.

The study authors said that such changes were not dangerous. Other experts have also noted that the benefits of getting vaccinated far outweigh any of the possible side effects.

“The impact of vaccine hesitancy cannot be underestimated in public health,” said Dr. Alison Edelman, a member of the American College of Obstetricians and Gynecologists’ Clinical Document Review Panel-Gynecology and expert on COVID-19 and menstruation changes.

“We have a long history of safe vaccine development, and we know a lot about vaccines — not all side effects with a vaccine are harmful like feeling temporarily fatigued, having a sore arm, or getting a fever. But if we didn’t counsel individuals on what to expect, then those experiencing that side effect would have been really concerned, and that fuels fear and mistrust.”

Previous studies found the most commonly reported side effect after taking the COVID vaccine was heavy period. However, this “quickly reversed” in a few cycles.

How the vaccine can do this is a different story. According to study author Kathryn Clancy, a professor of anthropology at the University of Illinois Urbana-Champaign, since the uterus is an immune organ, it’s not hard to imagine why it would be affected.