Changes to menstrual bleeding patterns after COVID-19 vaccination are not uncommon, according to a study published in Science Advances. Noting that people began sharing experiences of unexpected menstrual bleeding after COVID-19 vaccination early in 2021, Katharine M. N. Lee, PhD, MS, and colleagues examined the emerging phenomenon of changed menstrual bleeding patterns using a Web-based survey. Data were included from 39,129 respondents, aged 18-80: 90.9% of respondents were woman-only identifying and 9.1% were gender-diverse respondents. After vaccination, 42% of people with regular menstrual cycles bled more heavily than usual, while 44% reported no change. Of the respondents who do not typically menstruate, breakthrough bleeding was reported by 71%, 39%, and 66% of those on long-acting reversible contraceptives, those taking gender-affirming hormones, and postmenopausal people, 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.