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.