Women receiving one dose of a awarded to five institutions to explore potential links between COVID-19 vaccination and menstrual changes.
The study authors analyzed de-identified data from a fertility tracking app, Natural Cycles. Users input data on their temperature and their menstrual cycles and can consent to the use of their de-identified data for research. For vaccinated individuals, data was from three consecutive cycles before vaccination and from three more consecutive cycles, including the cycle or cycles in which vaccination took place. For unvaccinated individuals, data was collected for six consecutive cycles. Of the 3,959 individuals in the study, 2,403 were vaccinated and 1,556 were unvaccinated.
Most vaccinated users received the Pfizer and Moderna vaccines. On average, the first vaccination dose was associated with a .71-day cycle increase in cycle length and the second dose with a .91-day increase. Therefore, users vaccinated over two cycles had an increase of less than one day in each of the vaccination cycles. There were no changes in the number of menstrual bleeding days for the vaccinated individuals. The researchers saw no significant change in cycle length for the unvaccinated app users.
A subgroup of app users who received two vaccine doses in the same menstrual cycle (358 users) had a larger average increase in cycle length of two days. However, this change appears to decrease in subsequent cycles, indicating that the menstrual changes likely are temporary. The authors added that the International Federation of Gynecology and Obstetrics classifies a variation in cycle length as normal if the change is less than eight days.
Reference: “Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination: A U.S. Cohort” by Edelman, A. et al., 6 January 2022, Obstetrics & Gynecology.
About the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): NICHD leads research and training to understand human development, improve reproductive health, enhance the lives of children and adolescents, and optimize abilities for all.
About the National Institutes of Health (NIH): NIH, the nation’s medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases.