A large international study has confirmed the results of a previous US study that linked vaccination against COVID-19 with an average increase in menstrual cycle length.
The increase, although less than a day, was consistent in data from nearly 20,000 people in Canada, the UK and other parts of Europe and the world. The study was originally published in the British Medical Journal.
“These results provide additional information to advise women on what to expect after vaccination,” said Diana Bianchi, MD, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) of the National Institutes of Heath in a statement. “Changes after vaccination appear to be small, within the normal range of variation, and temporary.”
Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University, led the study. Talk with The Washington Postshe said the effects discovered by her team were temporary and no indication was found that they affected fertility.
“Now we can give people information about what to expect with menstrual cycles,” Edelman said. “So hopefully that’s overall very reassuring for people.”
Although Edelman and her team still don’t know exactly why vaccines seem to affect menstrual cycles, she said the immune and reproductive systems are linked. She pointed to the parameters of her own study at the point of sale, acknowledging that her study did not include people on birth control. It also only used data from people who had regular cycles before being vaccinated and who were between the ages of 18 and 45.
The study was funded by the NIH and the National Institute of Child Health and Human Development as part of $1.67 million awarded to five institutions to explore potential links between COVID-19 vaccination and changes menstrual.
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The study found that participants who received one of nine different vaccines against the virus had an increased number of days during their periods, but the increase resolved into cycles after the vaccinations. “De-identified” data from the Natural Cycles period-tracking app was analyzed in both studies.
Of the thousands of participants, 14,936 were vaccinated, while 4,686 were not. The researchers were able to analyze data that app users entered about their menstrual cycles each month, looking at three cycles before vaccination and one cycle after. They then compared the information to the unvaccinated group.
The data revealed that vaccinated people had their periods, on average, 0.71 days late after the first dose of a vaccine. People who had two injections in one menstrual cycle experienced the most disruption, increasing cycles by four days, with a 13% delay of eight days or more.
The authors called for future studies to be conducted on aspects of vaccination-related changes in menstrual cycles. They said further investigation was needed into the unexpected bleeding as well as changes in flow and pain levels.
After anecdotal reports were posted on social media by people reporting changes in their menstrual cycles, NIH-funded research into possible links between the two, with the first study published in April in the journal Obstetrics and Gynecology.
The researchers also pulled this data from the Natural Cycles app, which helps people track their menstrual cycles, and found that vaccines were associated with less than a day’s change in menstrual cycle length or a change from when they started — “no clinically meaningful changes at the population level,” they wrote.
As information on the coronavirus pandemic changes quickly, PEOPLE is committed to providing the most up-to-date data in our coverage. Some of the information in this story may have changed after publication. For the latest news on COVID-19, readers are encouraged to use the online resources of the CDC, WHO and local public health departments.