CNN
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People who received the Covid-19 vaccine reported slightly longer menstrual cycles, but the change was temporary, according to a new study published Tuesday in the journal BMJ Medicine.
“Compared to the unvaccinated group, vaccinated individuals had an adjusted menstrual cycle increase of less than one day with both the first and second vaccine doses,” wrote the study authors, from universities in Oregon, Massachusetts, London and Edinburgh. “Individuals 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 the unvaccinated.”
The vaccinated cohort had a change in cycle length of about 0.71 days after their first vaccine dose, the authors said.
There was also a “significant increase” in the number of respondents who reported an increase of more than eight days in their cycle length, the authors said, with 13.5% of vaccinated individuals and 5% of unvaccinated participants.
“We found no differences in menstrual length in a group of vaccinated individuals, compared to the unvaccinated cohort,” the study says.
The changes in cycle length did not persist in the cycle after vaccination, the authors said — except in the group that received two doses in one cycle.
The changes seemed similar regardless of 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 vaccinated cohort, they looked at three pre-vaccination cycles and at least the first Covid-19 vaccine dose cycle. For the unvaccinated, they include four to six cycles from a comparable period.
The participants came from a number of countries, but mainly from Europe and the US and Canada. Nearly two-thirds of the 15,000 participants vaccinated had received the Pfizer/BioNTech vaccine, although the participants had also received the Johnson & Johnson, Moderna and Oxford-AstraZeneca vaccines, among others.
“These findings provide additional information for advising 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), said in a press release. “Changes after vaccination appear small, within the normal range of variation, and transient.”
The study has some limitations, the authors say, including that people using hormonal birth control 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 participants.
The findings are consistent with a previous study published by the same group of researchers that focused on women in the US.
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