Menstrual changes after getting COVID-19 vaccines may be far more common than previously known, according to an analysis published Friday in the journal Science Advances, NBC News reports.
A study of over 39,000 people 18 to 80 years old who were fully vaccinated and had not contracted COVID found that 42% of respondents with regular menstrual cycles said they bled more heavily than usual after vaccination. Meanwhile, 44% reported no change and around 14% reported a lighter period. Among those who don’t typically have their period due to long-term contraceptives, gender-affirming hormone treatments or menopause, the study suggests many experienced breakthrough or unexpected bleeding after their COVID shots.
The study authors cautioned, though, that the percentages do not necessarily represent the rate of menstrual changes in the general population, since people who observed a difference were more likely to participate. The survey’s aim was simply to provide evidence for future studies, not to establish cause and effect. Other recent research also found that the COVID vaccine is associated with a small change in menstrual cycle length.
Study co-author Katherine M.N. Lee and her colleagues were inspired to ask people about their menstruation cycles after being vaccinated and seeing both friends and strangers online wonder why they experienced an unexpected change. However, these anecdotes were at the time met with the rebuttal that there was no data linking menstrual changes to vaccination. Overall, most pharmaceutical trials don't included questions about changes to menstruation, something Lee blames on the lack of senior people in science and medicine who are not "white men."
It’s not yet understood why menstrual changes happen after vaccination. But experts suggest the answer will likely stem from the overlap between the immune system and endocrine system, which plays a role in reproduction.
Still, COVID itself has been shown to disrupt menstruation more significantly than vaccines. This may be because of an ongoing immune reaction and the lifestyle changes that come with being sick. People with long COVID are especially affected and more research on the reason is needed.