COVID-19 vaccines linked to temporary changes in menstrual cycle, new study confirms

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Researchers have been observing the various effects of the COVID-19 vaccine on menstrual cycles since many menstruating people began reporting changes following vaccination. Now, a new global study confirms the link between a temporary increase in the menstrual cycles of some individuals as a result of the COVID-19 vaccine.

The study, conducted by the U.S.-based National Institute of Health, found that the increase in cycle length resolved for most of the nearly 20,000 study participants following vaccination.

The international study included participants from Canada, the United Kingdom, the United States, Europe, and other areas of the globe who received any of nine different COVID-19 vaccines.

The study examined at least three consecutive menstrual cycles prior to vaccination, and at least one cycle following. Of the 19,622 participants, 14,936 were vaccinated and 4,686 were not.  For the unvaccinated participants, the researchers looked at at least four consecutive cycles of a similar interval.

The study reported that those who were vaccinated experienced an increase of less than one day in each cycle in which they were vaccinated, citing on average a .71 day increase after the first dose and a .56 day average increase after the second dose.

Meanwhile, participants who received two vaccine doses within a single cycle experienced a lengthier increase at 3.91 day average increase in cycle length.

Compared to those who were not vaccinated, those who received one dose per cycle only experienced an increase by .02 days, while a further increase was reported for those who received two doses in one cycle at an increase of .85 days, in the time period studied following vaccination.

“In respect to changes to menstrual cycle length, stress, both infectious and non-infectious, can impact women's cycle, which is a necessarily sensitive barometer when something is off,” says Shirin Kalyan, adjunct professor of the division of endocrinology and metabolism at the University of British Columbia.

“It is reassuring the changes that have been noted appear to be mostly short lived, so I don't want anyone to be unnecessarily worried.”

Kalyan says that more research needs to be done to understand this phenomenon.

“We need to look into signals when they arise - which is what the researchers in the recently published study in BMJ Medicine did. This is especially relevant for new technologies, like the mRNA delivery platform for COVID-19, for which we have had very little relative clinical experience prior to this pandemic.”

While a change in menstrual cycle length under eight days is considered normal and may be of low interest to health-care professionals and researchers, the study says that “perceived changes in a bodily function linked to fertility may be alarming to those experiencing it and could contribute to vaccine hesitancy.”

Notably, separate studies have found no connection between COVID-19 vaccinations and risk of infertility.

In August, a different study found that some individuals who received the COVID-19 vaccination temporarily experienced heavier bleeding than usual.