When clinical trials began testing the various COVID-19 vaccines, one question participants weren’t asked about was whether they experienced any changes to their menstrual cycles or unexpected bleeding after getting vaccinated.
Soon after vaccinations became available, many people started talking with their physicians about symptoms. Many noticed an increase in their period length. Initial results were not encouraging, according to the researchers of the new study. Science Advances. “In media coverage, medical doctors and public health experts hastened to say that there was ‘no biological mechanism’ or ‘no data’ to support a relationship between vaccine administration and menstrual changes,” the authors write. “In other cases, experts declared that these changes were more likely a result of ‘stress.’” But these types of changes are not unheard of: vaccines for typhoid, Hepatitis B, and HPV have sometimes been associated with menstrual irregularities.
The researchers conducted a survey to gain an even better idea of the situation in April 2021. Over 39,000 people participated in the survey. 91% identified themselves as females and 9% were gender-diverse. Among those who have regular periods, 41% of respondents reported heavier bleeding after getting vaccinated, while 44% said they didn’t notice any change. People who don’t usually menstruate reported breakthrough bleeding by 71% of those on long-acting contraceptives and 39% for people taking gender-affirming hormonal therapy. 66% also reported it.
Older people and those who were classified as Hispanic/Latinx or non-white had higher chances of experiencing a heavy flow after vaccination. This was also true for individuals who suffered from fever, fatigue, endometriosis or menorrhagia.
It’s too soon for researchers to make any conclusions about what the results might mean; the study relied on self-reported experiences, which presents challenges. People who experienced menstrual change may have responded more readily to the survey. Researchers can’t yet say that the vaccine caused these changes—and if it did, exactly how or why. However, one theory holds that this is due to how immune systems respond to the vaccine. As the study notes: “Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine.”
“We suspect that for most people the changes associated with COVID-19 vaccination are short-term, and we encourage anyone who is worried to contact their doctor for further care,” said co-author Katharine Lee, an anthropology professor at Tulane University, in a press release. “We want to reiterate that getting the vaccine is one of the best ways to prevent getting very sick with COVID, and we know that having COVID itself can lead not only to changes in periods, but also hospitalization, long COVID and death.”
Here are more must-read stories from TIME