(CNN) — People who received the covid-19 vaccine reported having slightly longer menstrual cycles, but the change was temporary, according to a new study published this Tuesday in the journal BMJ Medicine.
“Compared to the unvaccinated group, vaccinated individuals had an adjusted increase in menstrual cycle length of less than a day with the first and second doses of the vaccine,” the study authors, from the Universities of Oregon, wrote. , Massachusetts, London and Edinburgh. “People who received two doses of a COVID-19 vaccine in a single cycle had an adjusted increase in cycle length of 3.70 days compared to unvaccinated people.”
The base vaccinated population had a change in cycle length of about 0.71 days after the first dose of the vaccine, the authors said.
There was also a “significant increase” in the rate of people surveyed who had an increase of more than eight days in their cycle length, the authors said, finding that 13.5% of those vaccinated and 5% of unvaccinated participants reported this.
“We found no difference in the duration of menstruation in any group of vaccinated individuals, compared to the base population not vaccinated,” says the study.
Changes in cycle length were not maintained in the post-vaccination cycle, the authors said, except in the group that received two doses in one cycle.
The changes appeared to be similar no matter which vaccine a person received.
The authors looked at nearly 20,000 people, representing more than 250,000 cycles, who recorded their data using the Natural Cycles app between October 1, 2020, and November 7, 2021. For the base vaccinated population, they looked at three cycles prior to the vaccine and at least the first dose cycle of the covid-19 vaccine. For the unvaccinated, they included four to six cycles of a similar time period.
The participants came from various countries, but mostly from Europe, the United States and Canada. Nearly two-thirds of the 15,000 vaccinated participants had received the Pfizer/BioNTech vaccine, although the participants had also received vaccines from Johnson & Johnson, Moderna, and Oxford-AstraZeneca, among others.
“These findings provide additional information to advise women on what to expect after vaccination,” said Dr. Diana Bianchi, director of the National Institute of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), in a press release. “The post-vaccination changes appear to be small, within the normal range of variation, and temporary.”
The research has some limitations, the authors say, including that people using hormonal contraceptives were not participants, the researchers were limited in the number of post-vaccine cycles, and they were unable to account for the effects of possible infections in the participants. .
The findings align with an earlier study published by the same group of researchers that focused on women in the United States.