Women’s periods may be late after coronavirus vaccination, study suggests

An analysis of thousands of menstrual records offers support for anecdotal reports of erratic cycles after shots.

A woman receives a COVID-19 vaccine injection by a pharmacist at a clinic in Lawrence, Mass., on Wednesday, Dec. 29, 2021, Research published Wednesday, Jan. 6, 2022, tracked nearly 4,000 U.S. women through six menstrual cycles and on average, the next period after a shot started about a day later than usual. But there was no change in the number of days of menstrual bleeding after COVID-19 vaccination. AP Photo/Charles Krupa

Shortly after coronavirus vaccines were rolled out, women started reporting erratic menstrual cycles after receiving the shots.

Some said their periods were late. Others reported heavier bleeding than usual or painful bleeding. Some postmenopausal women even said they had menstruated again.

A study published Thursday found that women’s menstrual cycles did indeed change after vaccination against the coronavirus. The authors reported that women who were inoculated had slightly longer menstrual cycles after receiving the vaccine than those who were not vaccinated.

Their periods, which came almost a day later on average, were not prolonged, however, and the effect was transient, with cycle lengths bouncing back to normal within one or two months.


The study, in the journal Obstetrics & Gynecology, is one of the first to support anecdotal reports from women that their menstrual cycles were off after vaccination, said Dr. Hugh Taylor, chair of the department of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine.

“It validates that there is something real here,” Taylor said.

One drawback of the study, which focused on U.S. residents, is that the sample is not nationally representative and cannot be generalized to the population at large.

The data was provided by a company called Natural Cycles that makes an app to track fertility. Its users are more likely to be white and college-educated than the U.S. population overall; they are also thinner than the average American woman — weight can affect menstruation — and do not use hormonal contraception.

The study was carried out by researchers at Oregon Health & Science University and the Warren Alpert Medical School of Brown University, in collaboration with investigators from Natural Cycles.

Researchers looked at records from nearly 4,000 women who had tracked their menstruation, including about 2,400 who were vaccinated and about 1,550 who were not. All were U.S. residents ages 18 to 45 who had logged their periods for at least six months.


For those who were vaccinated, researchers examined the three cycles before and after the vaccine to look for changes, comparing them with a similar six-month duration in women who did not receive a vaccination.

Overall, vaccination was associated with less than a full day’s change in cycle length, on average, after both vaccine doses, compared with pre-vaccine cycles. The unvaccinated group saw no significant changes over the six months.

This article originally appeared in The New York Times.


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