The researchers found that after vaccination, 42 percent of people with regular menstrual cycles bled more heavily than usual, while 44 percent reported no change. Of the respondents who do not typically menstruate, 71, 39, and 66 percent of those on long-acting reversible contraceptives, those taking gender-affirming hormones, and postmenopausal people reported breakthrough bleeding, respectively. Significant associations were seen for increased/breakthrough bleeding with age, systemic vaccine side effects (fever and/or fatigue), history of pregnancy or birth, and ethnicity.
Noting that people began sharing experiences of unexpected menstrual bleeding after COVID-19 vaccination early in 2021, Katherine M.N. Lee, Ph.D., from the Washington University School of Medicine in St. Louis, and colleagues examined the emerging phenomenon of changed menstrual bleeding patterns among currently and formerly menstruating people using a web-based survey. Data were included from 39,129 respondents, aged 18 to 80 years: 90.9 percent of respondents were woman-only identifying and 9.1 percent were gender-diverse respondents.