A large study funded by the National Institutes of Health has confirmed previous findings that coronavirus vaccination can change the timing of when women get their periods, a fact that was once dismissed as "misinformation."
Vaccinated women on average experienced an increase of less than one day in their menstrual cycle, compared to those who did not receive a COVID-19 shot. The change appeared to be temporary and resolved one cycle after vaccination, according to the study published by BMJ Medicine.
“These findings provide additional information for counseling women on what to expect after vaccination,” Dr. Diana Bianchi, director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, said in a statement. "Changes following vaccination appear to be small, within the normal range of variation, and temporary.”
The study led by Dr. Alison Edelman at Oregon Health & Science University was funded by a $1.67 million research grant to explore potential links between COVID-19 vaccination and menstrual changes.
Researchers examined de-identified data from nearly 20,000 international women who participated using a fertility tracking app called Natural Cycles. App users provided information on their temperature and their menstrual cycles and consented to share that information anonymously to help scientists conduct their research. Most of the women were from the United Kingdom, the U.S., Canada, and Europe.
Vaccinated women in the study took both the mRNA vaccines manufactured by Pfizer-BioNTech and Moderna, as well as the engineered vaccines produced by AstraZeneca, Covishield, Janssen/Johnson & Johnson and Sputnik, and the inactivated virus vaccines by Covaxin, Sinopharm, and Sinovac.
Of the 19,622 total participants, 14,936 were vaccinated and 4,686 who were not.
Researchers found that vaccinated women saw an average cycle increase of .71 days after their first vaccine dose and a .56-day increase after the second dose. Women who received two doses of a COVID-19 vaccine within a single cycle had a 3.91-day increase in their cycle.
The changes did not differ based on the type of vaccine received.
"A change in cycle length of less than eight days is considered within the normal range of variation," NIH said in a press release. "Although small menstrual changes may not be meaningful to health care professionals and researchers, the study authors wrote, perceived changes in a bodily function linked to fertility may be alarming to those experiencing it and could contribute to vaccine hesitancy," the agency cautioned.
The large international study confirmed the results of a previous U.S.-based study that linked changes in women's menstrual cycles to COVID-19 vaccination.
Before this research was published, public health officials downplayed concerns from women who said they felt their periods change after receiving a COVID-19 vaccine.
In April 2021, Alice Lu-Culligan and Dr. Randi Hutter Epstein at Yale School of Medicine wrote in the New York Times that there was "no data linking the vaccines to changes in menstruation" at the time.
The San Francisco Chronicle labeled "claims that vaccines may affect women’s menstrual cycles" as "myths and misinformation" in a May 2021 article.
Later in January 2022, the NIH acknowledged that COVID-19 vaccines were "linked to small increase in menstrual cycle length."
The U.S. Centers for Disease Control and Prevention does not list changes in women's menstrual cycles as a possible side effect of COVID-19 vaccination.