Impact of COVID Vaccine On Menstrual Cycle Minimal, Resolves Quickly: Study

While COVID-19 vaccines did influence menstrual periods for some persons, according to a study released Thursday by Oregon Health and Science University, the impact was minimal and often cleared by the next month.

The findings, according to Dr. Alison Edelman, the study's lead author, are both encouraging for individuals concerned about the vaccine altering their cycles and validating for those who have noticed some changes.

The study followed roughly 4,000 women in the U.S. who had "the most normal of normal" cycles.

It followed vaccinated and unvaccinated women for a total of six menstrual cycles, which are measured from the first day of one period to the first day of the next. The tracking of vaccinated women's samples included the month of vaccination in the middle of their sample, as well as three cycles before and after.

The average period following vaccination began one or two days later than usual in the sample of vaccinated women, with no change in the number of days the women experienced menstrual bleeding.

There was a significantly higher difference in the length of the next cycle for around 350 women who received both doses within the time frame tested, which was about two days. In the following months, only around 10% of that subset suffered a shift of at least eight days, although their cycles mainly reverted to normal.

Edelman's team looked at data from Natural Cycles, a birth control app approved by the Food and Drug Administration for women to track their menstrual cycles and predict when they'll become pregnant.

Stress, nutrition, and even exercise can cause transitory alterations from month to month. Slight variations are common.

According to Edelman, one explanation is that when the immune system is activated at specific points during the cycle, "our body clock or what controls the menstrual cycle can have a hiccup."

The findings are comparable with data from a still-unpublished study in the United Kingdom, which found no significant menstrual abnormalities linked to immunization.

Longer, heavier periods were always a possibility as a result of COVID-19 vaccination. Any stress on the immune system can influence the menstrual cycle, but this does not imply any effect on fertility. (The vaccine has no impact on fertility or pregnancy.)

Last spring, the National Institutes of Health (NIH) issued a request for study proposals that may investigate the influence on periods.

Edelman said the team plans to look at data on the heaviness of people's periods following vaccination as well. They also intend to examine data from Natural Cycles from people in different nations. Other NIH-funded research teams are moving forward with their studies on menstrual cycles and the COVID-19 vaccine, with results expected to be published soon.