COVID-19 vaccination can affect menstrual cycle, global study confirms

A new study involving nearly 20,000 participants around the world found that people got their periods about a day late, on average, after receiving their first dose of the COVID-19 vaccine. 

The findings, published Sept. 27 in the British Medical Journal, are based on de-identified data from a fertility-tracking app called Natural Cycles. Researchers used de-identified data from 14,936 vaccinated participants and 4,686 unvaccinated participants. Findings showed that vaccinated people got their periods 0.71 days late, on average, after the first dose of vaccine. People who received both of their vaccine doses in a single cycle had a 3.91 day average increase in cycle length. 

For most people, the effects were temporary and lasted one cycle before returning to normal, Alison Edelman, MD, study author and professor of obstetrics and gynecology at Oregon Health & Science University in Portland, told The Washington Post.

"Now we can give people information about possibly what to expect with menstrual cycles," Dr. Edelman said. "So I hope that's overall really reassuring to individuals," she said, adding that there were no indications the side effects had any effect on fertility. 

The findings support the claims of thousands of Americans who earlier in the pandemic reported unexpected menstrual changes around the time they received a vaccine. In January, a smaller study involving nearly 4,000 women found women's menstrual cycles were slightly longer after receiving the vaccine compared to unvaccinated women. 

A cycle length change of less than eight days is considered within the normal range of variation, the National Institutes of Health said in a news release on the latest findings. "Although small menstrual changes may not be meaningful to healthcare professionals and researchers … perceived changes in a bodily function linked to fertility may be alarming to those experiencing it and could contribute to vaccine hesitancy," the NIH said. 

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