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Having COVID-19 was linked to a slight, temporary change in menstrual cycle length similar to changes seen after COVID-19 vaccination, according to a study of self-reported data from a menstrual tracking app.

People who experienced COVID-19 had a 1.45-day adjusted increase in cycle length during COVID-19 infection compared with the three cycles before infection (95% CI 0.86-2.04), while those who were vaccinated against COVID-19 had a 1.14-day adjusted increase in cycle length after being vaccinated compared to previous cycles (95% CI 0.60-1.69).

Although both those changes were more than the 0.68-day decrease in cycle length seen in the control group of people who were neither vaccinated nor reported to have COVID-19, the vaccinated and infected groups did not differ significantly from each other.

And for both groups, the cycle changes disappeared by the next cycle, reported Alexandra Alvergne, PhD, of the Institute for Evolutionary Sciences at Montpellier University in France, and co-authors published in .

"The change in cycle length was minimal and limited to only the cycle of either illness or vaccination," commented Pamela Berens, MD, an ob/gyn at the McGovern Medical School at UTHealth Houston, who was not involved in the study. "[This research] might provide some reassurance to patients who are concerned about the impact of COVID-19 or vaccination on their cycles."

Study co-author Alison Edelman, MD, MPH, of the Oregon Health & Science University School of Medicine in Portland, said this research contributes a large dataset on COVID-19 and menstrual cycle changes, which has received less attention compared with the scrutiny around COVID-19 vaccination.

"We wanted to come back around to COVID-19 infection and see if that affected the menstrual cycle in similar ways," Edelman said.

Ultimately, the menstrual cycle changes "were small in magnitude and not clinically significant at the population level," her group concluded.

However, a small proportion of people did have a clinically significant change of more than 8 days. That proportion was higher for individuals with COVID-19 than for those in the vaccination or control groups (9.7%, 6.3%, and 6.9%, respectively), the authors noted. "COVID-19 vaccination at least 3 months before the onset of COVID-19 symptoms was protective for COVID-19–associated changes in cycle length," they wrote.

Next steps for researchers involve figuring out why COVID-19 infection causes changes in menstrual cycles.

"People want to know when things change and why they change," Edelman said adding that the findings are another tool for counseling patients and giving reassurance that "for the majority of individuals things get back to normal pretty quickly. And if they don't, they should be talking to their healthcare provider."

Researchers collected data from period tracking app as well as from surveys on COVID-19 vaccination and infection. About 13% of people who saw the survey in the Clue app clicked on the link, which is on par with other in-app surveys. Participants who consented gave prospective access to their menstrual cycle data. The survey asked about COVID-19 vaccination status, COVID-19 history, age, BMI, and country of residence.

In all, 6,514 people from 110 countries (half from the U.S.) met inclusion criteria and were included in the study. The sample included five menstrual cycles per person, totalling 32,570 cycles.

Participants were organized into three categories: a control group of 421 people who had no history of COVID-19 vaccination or infection, 1,450 who were vaccinated but never had COVID-19, and 4,643 who had COVID-19, further broken into vaccinated and unvaccinated. People who used hormonal contraceptives between 2019 and the survey were excluded, as were people older than 45, and anyone who didn't give consistent data. If a user flagged a cycle as abnormal, researchers removed it (n=10,788).

Initial COVID-19 symptoms occurred from January 2020 until the end of June 2022. COVID-19 vaccinations first began on Dec. 11, 2020, and were recorded through the end of July 2022. Researchers tracked within-user change in menstrual cycle length.

The fact that research relied on self-reported data from app users was a key limitation, although the authors also noted that self-reports of COVID-19 symptoms were fairly accurate earlier in the pandemic when data collection occurred.

Other limitations included the likelihood of asymptomatic cases of COVID-19 in the control cohort, and the inability to determine how specific variants could have different effects on menstrual cycles.

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