Impact of COVID Vaccines on Menstrual Cycles

Andrew M. Kaunitz, MD


January 21, 2022

This transcript has been edited for clarity.

Soon after COVID-19 vaccinations became available, anecdotal reports of changes in menstrual cycles appeared and were amplified by social media.

In a report published in the January 2022 issue of ACOG's "Green Journal," which received extensive media attention, investigators used data from a widely used fertility-awareness app known as Natural Cycles.

This study assessed whether COVID vaccination is associated with changes in menstrual cycle characteristics in US women age 18-45 years who reported regular cycles and were not using hormonal contraceptives.

The report utilized data collected between October 2020 and September 2021, and compared menstrual cycle changes among vaccinated individuals for three cycles prior to, and three cycles following, vaccination, with data from six cycles in unvaccinated women.

The app prompted users to report receipt or nonreceipt of COVID vaccination, including vaccine type. Based on International Federation of Gynecology and Obstetrics criteria, changes of less than 8 days in cycle length were considered normal.

Among almost 4000 women with evaluable menstrual cycle data, more than half were vaccinated during the study period.

Overall, receipt of vaccination was associated with a less than 1-day increase in cycle length, and the proportion of women experiencing changes in cycle length of 8 days or more was 4%-5%. This was similar among vaccinated and unvaccinated women.

In a subgroup analysis focusing on women who received two vaccinations within one menstrual cycle, the increase in length of their next cycle was 2 days. However, within two subsequent cycles, cycle length became similar in vaccinated and unvaccinated women.

When women who received their second shot in a subsequent cycle were analyzed, vaccination was no longer associated with significant differences in menstrual cycle length.

Vaccination brand was not associated with alterations in these findings. Likewise, vaccination was not associated with changes in duration of menstrual flow.

Limitations of this study include the fact that a high proportion of the women using this menstrual app were White and well educated, and that no information related to receipt of booster shots was collected.

Overall, this carefully conducted study validates anecdotal reports that vaccination causes changes in menstrual cycle length. However, women considering the COVID shot can be reassured that vaccination does not result in clinically significant or long-term changes in menstruation.

I am Andrew Kaunitz. Please take care of yourself and others.

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