COVID-19 Vaccination Alters Menstrual Cycle Length

As the COVID-19 pandemic continues and vaccinations continue to be advanced, many patients are concerned about vaccine side effects. Multiple sources have perpetuated the misconception that COVID-19 vaccination can cause infertility. Researchers in BMJ Medicine analyzed the effects of vaccination on menstruation and concluded that vaccination could alter cycle length for a small portion of menstruating individuals.

Using data collected from Natural Cycles, a menstrual cycle tracking app, researchers analyzed data from 19,622 menstruating individuals 18–45 years old. Of those participants, 14,936 were vaccinated, and 4,686 were unvaccinated. Patients studied had a minimum of three consecutive cycles before vaccination that lasted between 24 and 38 days or, for unvaccinated participants, four consecutive menstrual cycles during the same time.

The difference between cycles in vaccinated and unvaccinated individuals was less than one day. Despite that, individuals who received the first and second dose of the COVID-19 vaccine had a more significant increase in cycle length. Comparatively, the average increase in menstrual cycle length between vaccinated and unvaccinated individuals was 0.71 days. In contrast, those who received both doses during one cycle had a 3.70 day increase in cycle length.

Researchers in the publication state, “although we do find menstrual changes after COVID-19 vaccination, these changes are small compared with normal variation and resolve in the cycle after vaccination, except in people who received both doses in one menstrual cycle. Future work should assess other aspects of changes to menstrual cycles, such as unexpected vaginal bleeding, menstrual flow, and pain, and define the mechanism by which the postvaccination menstrual changes described here occur.”

Providers can use this research to better counsel patients with concerns about COVID-19 vaccinations. Informing patients that the changes in the menstrual cycle are minute and temporary may alleviate some hesitancy surrounding vaccinations. Beyond this information, it is important for providers to acknowledge that many other widely accepted medical interventions, such as blood thinners, antidepressants, and chemotherapy, also alter menstrual cycles. The COVID-19 vaccine has been rigorously tested and deemed as safe as these other interventional approaches.

Additionally, understanding the scope of menstrual cycle impact may inform providers to care for patients with menstrual cycle concerns. Discerning the extent to which vaccination can impact cycle length allows providers to determine what tests are necessary more effectively.