Premenopausal females who were vaccinated for COVID-19 were no more likely to report irregular menstrual cycles, heavier bleeds or menstrual pain than a similar group of unvaccinated women, according to a study funded by the National Institutes of Health. However, consistent with several previous studies, participants reported a small increase in cycle length of roughly one day following injection. The study, conducted by Amelia K. Wesselink, Ph.D., and Lauren A. Wise, Sc.D., of Boston University, and their colleagues, appears in Vaccine.
Background
Researchers analyzed questionnaire results of more than 1,000 participants trying to conceive without the use of hormones or fertility treatments, comparing responses of 437 participants who received at least one vaccine dose during the study period to those of 700 unvaccinated participants. Originally designed to identify risk factors for subfertility and miscarriage, the study 
asked participants to respond to online questionnaires every 8 weeks for up to 12 months on menstrual cycle characteristics, pregnancy status and medical factors. Participants ranged from 21 to 45 years of age.
Results
Compared to unvaccinated participants over a similar interval, those who received the first vaccine dose had cycles an average of 1.1 day longer after the first dose and 1.3 days longer after the second dose. The increase was temporary and did not occur in the second cycle after vaccination. The researchers did not find strong associations between vaccination and other menstrual cycle characteristics, including irregular cycles, long bleeds, heavy bleeds, or menstrual pain. Similarly, the authors did not find associations between the type of COVID-19 vaccines examined (Pfizer vs. Moderna) and cycle length.
Significance
The authors theorized that cycle length increases likely were due to immune response after vaccination rather than to any individual component of a particular vaccine.
Reference
Wesselink, A.K. COVID-19 vaccination and menstrual cycle characteristics: A prospective cohort study. Vaccine. 2023. DOI: 10.1016/j.vaccine.2023.06.012
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