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Last spring, amid the widespread rollout of coronavirus vaccines in the United States, some menstruators complained of altered menstrual cycles after vaccination. Those stories led the National Institutes of Health to award in August more than $1.6 million in funding to researchers to study possible links between coronavirus vaccines and changes to people’s menstrual cycles. On Thursday, the first such study was published in the journal Obstetrics and Gynecology.

The study found that coronavirus vaccinations are associated with a change of less than one day in menstrual cycle length, or the timing of when periods arrive over an average 28-day period, but that they’re not associated with any changes in length of actual bleeding.

Of the more than 3,900 Americans included in the study — about 2,400 of whom were vaccinated and about 1,500 of whom were unvaccinated — the vaccinated cohort got their periods less than one day later than they had pre-vaccination. A subset of individuals who received two mRNA vaccine doses in a single menstrual cycle had an average two-day increase in cycle length — meaning their period arrived about two days later than it had pre-vaccination — compared to unvaccinated individuals, who saw “no significant change” to their menstrual cycles over the course of the study.

The findings, the study notes, “cannot be explained by generalized pandemic stress” as some doctors and researchers previously speculated, given that the unvaccinated control group saw no changes to their menstrual cycle over the course of the study. Instead, the findings support — but do not definitively prove — the hypothesis that mRNA vaccinations create an immune response that could temporarily affect the hypothalamic-pituitary-ovarian axis, which regulates menstrual cycle timing.

The study’s lead author, Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University, characterized the results to The Lily as “incredibly reassuring at a population level,” adding that they were also “validating for people who’ve experienced menstrual disturbance [following vaccination].”

Diana Bianchi, director of the National Institutes of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development — which, with the NIH’s Office of Research on Women’s Health, allocated the $1.6 million to fund Edelman’s study and similar ones at four other institutions — characterized the changes to menstrual cycle length that the study identified as “really trivial.” That’s because “normal variation in the menstrual cycle really can be up to eight days, and on average this was less than one day,” she said.

Indeed, the study found that a small proportion of both vaccinated and unvaccinated people “experienced a clinically significant change in cycle length of eight days or more” — 4.3 percent of unvaccinated participants and 5.2 percent of vaccinated participants. For the subset of vaccinated individuals who received two vaccine doses in one menstrual cycle, 10 percent experienced a change in cycle length of eight days or more, which reduced within two post-vaccine cycles, the study found.

The study is the first within the group of NIH-funded studies undertaken to examine possible linkages between covid-19 vaccination and menstruation to publish its findings, according to Bianchi. The agency allocated the funding following months of media coverage on the topic, prompted by social media discussions among people who menstruate who raised concerns about the vaccines’ possible impact on periods, which The Lily first covered in April.

The study published Thursday is also the first to examine possible correlations between covid-19 vaccinations and menstruation that tracked its cohorts prospectively, meaning both before and after vaccination, rather than retrospectively, or only after vaccination, as studies in the U.K. and Norway did — neither of which have been peer-reviewed. The U.S. study published Thursday went through an expedited review process, the study notes.

The U.S. research incorporated individuals between the ages of 18 to 45 who were tracking their menstrual cycles on the app Natural Cycles, the first FDA-approved birth control app. Using a thermometer provided by the app, users log their temperature and other data — including their periods, vaccination status, cervical mucus and sex drive — in the app to determine their daily fertility status.

The app does not collect gender identity data, according to Natural Cycles spokesperson Lauren Hanafin, meaning that it’s unclear whether, or how, many transgender or nonbinary people were incorporated in the study. The app informs users they cannot be using hormones that disrupt ovulation, according to Hanafin, and Edelman said the study was specifically undertaken to exclude participants on hormones given those disruptions.

Researchers analyzed six consecutive menstrual cycles for each user to calculate the mean change within users’ individual cycles and menses lengths over the course of the study: For vaccinated users, researchers used the three cycles before their first vaccine dose and three during or after vaccination, and for unvaccinated users, they used six cycles over a similar time period. Fifty-five percent of the vaccinated cohort received the Pfizer-BioNTech vaccine, while 35 percent received Moderna. Seven percent received Johnson & Johnson/Janssen, which is not an mRNA vaccine.

The prospective focus of the study is a key part of its strength, Bianchi told The Lily: “It was an exceptionally well-designed prospective study,” she said. “Prospective is key, because … a lot of the data that have come in from [people talking about their concerns on] social media, et cetera, was retrospective, and that is subject to recall bias.”

Limits of the study include that “it may not be generalizable to the U.S. population” given that Natural Cycles users are more likely to be White, college-educated and have lower body mass indexes than the rest of the population, and those studied are not using hormonal contraception, the study notes. Researchers also excluded users with irregular cycles and did not have data on covid-19 infections in either the vaccinated or unvaccinated cohorts included in the study, it notes.

Questions remain around possible linkages between covid-19 vaccination and menstrual symptoms, as well as the vaccines’ possible impact on quality and quantity of menstrual bleeding, the study notes. The other NIH-funded studies, which Bianchi expects will publish findings by next summer, will study these questions and others. She is “hoping that these studies will inspire vaccine manufacturers to include questions about menstrual health in future clinical trials,” she said.

Edelman and her team are working on a repeat analysis of the study using data from international Natural Cycle users, and they are using data from Natural Cycles to study menstrual flow characteristics before and after vaccination. They are also planning similar analyses based on data provided by Clu, another menstrual tracking app, Edelman said; they hope to have results from those studies by the spring.

But for now, Edelman sees her study’s recent findings as empowering for women and people who menstruate, given that many have complained of the lack of research around this issue.

“It’s validating for those that experienced or reported this, and I think it gives us a really concrete tool to tell people what to expect,” she said.

Bianchi agreed, adding that this study and others could go a long way toward reducing vaccine hesitancy in some people who menstruate.

“It’s the lack of information that creates, possibly, vaccine hesitancy,” she said. “So the power of this study is it really shows what to expect and not to be alarmed if you’re menstruating regularly every 30 days and day 30 comes [after vaccination] and you don’t have your period.”

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