When the COVID-19 vaccine became widely available last year, it led to reports from some women of changes to their menstrual cycles after getting vaccinated.
Thanks in part to the persistence of women who spoke out on social media and documented their side effects in an online database created by two researchers, the National Institutes of Health (NIH) announced in August it was committing $1.6 million in funding to launch research studies at five universities across the country.
Now less than six months later, one of those studies, led by researchers at Oregon Health and Science University, formally documents what women described experiencing.
The study, published Thursday in the journal Obstetrics & Gynecology, found that vaccination can cause temporary changes to the timing of menstruation, with the typical experience being a period that's late by about one day after vaccination.
The effects were temporary, with a normal menstrual cycle returning within one or two months after getting the shot, according to the study, which evaluated the cycles of nearly 4,000 women who use Natural Cycles, an FDA-approved birth control app.
Researchers described their findings as "reassuring" because they help affirm women's anecdotal experiences, offer additional reassurance that the changes are only temporary and let women know what to expect after getting vaccinated.
“This research is reassuring given the many questions around the impact of the COVID-19 vaccines on the menstrual cycle and the lack of evidence to answer these questions prior to our study,” Dr. Jack Pearson, Natural Cycles’ lead medical researcher and on-staff medical expert, said in a statement. “The menstrual cycle is often overlooked and under-researched, especially when considering what a key indicator it is for a woman’s overall health.”
Experts stress the study's findingsdo not mean the COVID-19 vaccine impacts current or future fertility.
There is also a growing body of research showing the vaccine is safe and effective for pregnant women. The most recent study, released last week, found no increased risk of preterm or low-weight birth among babies born to pregnant women who got a COVID-19 vaccine shot, compared to babies born to unvaccinated pregnant women.
Rather, getting vaccinated produces a strong immune system response in the days following the shot, which may cause temporary changes to menstrual cycles. Studies have also documented temporary menstrual cycle changes among women who get COVID-19 infections.
Menstrual changes are controlled by a part of the brain called the hypothalamus, along with the ovaries, which use hormones as signals. These hormone signals can be disrupted when the body goes through changes that occur with an infection and even a vaccine.
Temporary changes to the menstrual cycle should not be a concern for women, experts say. Changes lasting "three months consecutively, or more" are when health care providers typically make investigation or treatment plans, Dr. Jessica Shepherd, OBGYN and chief medical officer at Verywell Health, told ABC News last year.
Data on menstrual side effects was not widely collected during clinical trials for COVID-19 vaccines, which were conducted by the companies behind the vaccines, Pfizer and BioNTech, Moderna and Johnson & Johnson, Candace Tingen, Ph.D., program director of the Gynecologic Health and Disease Branch at the National Institute of Child Health and Human Development, told ABC News in September, shortly after the NIH's funding was announced.
She said the NIH was motivated to fund research both from reports of menstrual side effects as well as the misinformation that followed around menstrual changes and fertility.
"There was a lot of misinformation out there, and NIH sees its mandate as countering misinformation with accurate information," Tingen said. "[Research] is something that we could do to step in and provide some real information about whether or not this is this is accurate."
In addition to Oregon Health and Science University, NIH-funded research is currently underway at Boston University, Harvard Medical School, Johns Hopkins University and Michigan State University.
Researchers are studying everything from menstrual cycle changes reported on period tracking apps like Clue to menstrual changes in women with endometriosis and women trying to get pregnant. They are examining how the vaccines may have affected flow, cycle length and pain, as well as exploring why COVID-19 vaccines may cause changes, according to Tingen.
ABC News' Sony Salzman and Sara Yumeen, M.D., a preliminary-year internal medicine resident at Hartford Healthcare St. Vincent's Medical Center in Connecticut and a contributor to the ABC News Medical Unit, contributed to this report.