A study which surveyed 35,000 women, published in leading journal Science, has found that 42% of around 27,000 respondents who have regular periods bled more heavily than usual following Covid vaccination. Among around 4,000 participants who typically do not menstruate, 71% of those on hormone-based contraceptives, 39% of those on gender-affirming hormones and 66% of postmenopausal women reported breakthrough bleeding following Covid vaccination.
The study was based on a convenience sample obtained via an online survey promoted through social media and other channels, so the authors are clear rates of adverse events cannot be calculated from it as there may be a bias towards those who suffered the adverse events being studied. There also wasn’t an unvaccinated control group, though anyone who had had suspected or confirmed Covid was excluded. Nonetheless, the study shows that menstrual irregularities are very likely to be a real issue connected with the vaccines.
The authors note that other vaccines have been associated with effects on menstrual bleeding and that there are “multiple plausible biological mechanisms to explain a relationship between an acute immune challenge like a vaccine, its corresponding and well-known systemic effects on hemostasis and inflammation, and menstrual repair mechanisms of the uterus”. They suspect a mechanism via inflammation of the immune system rather than disruption of ovarian hormones because those taking hormone-based contraception were not affected differently.
They found that increased or breakthrough bleeding was significantly associated with age, experiencing systemic vaccine side-effects (i.e., fever or fatigue), having previously been pregnant, and ethnicity.
The authors write that signal detection in the U.S. Vaccine Adverse Event Reporting System (VAERS) has “resulted in the identification of several possible effects on menstruation that suggest that further research is needed”. They criticise vaccine trials for not including queries about changes in menstruation as part of standard safety monitoring and recommend further study.