A healthcare worker administers a dose of the Pfizer-BioNTech Covid-19 vaccine at a immunization clinic at the Peabody Institute Library in Peabody, Massachusetts, on Wednesday, January 26, 2022.
Vanessa Leroy | Bloomberg | Getty Images
According to a large international study funded by the National Institutes of Health, the Covid-19 vaccination is associated with a slight lengthening of a woman’s menstrual cycle, delaying the onset of bleeding by a few hours.
dr Diana Bianchi, director of the NIH Institute for Child Health and Human Development, said the post-vaccination changes were small, transient and within the normal range. However, the longer menstrual cycle, typically around a month long, didn’t necessarily increase the number of bleeding days, according to the health agency.
A change in menstrual cycle length of eight days or less is considered within the normal range of variation, the NIH said. The participants’ menstrual cycles lengthened by an average of 0.71 days, or less than 24 hours, after the first vaccine dose and by just over half a day after the second dose, according to the study’s results. In women who received both doses of the vaccine in a single menstrual period, their cycle lengthened by 3.91 days.
But more than 1,300 women saw their cycle lengthen by eight days or more, accounting for 6.2% of the vaccinated and 5% of the unvaccinated in the study. Younger women who had longer cycles prior to vaccination were more likely to have a greater delay in the start of their periods.
After the vaccination series was completed, cycle length had largely returned to normal in women who received one dose per menstrual cycle, and by around 20 hours in women who received both doses in a single cycle.
Nearly 20,000 people in Canada, UK, USA, Europe and other parts of the world took part in the study. Participants received one of nine different vaccines: Pfizer-BioNTech, Moderna, Johnson & Johnson, AstraZeneca, Covishield, Sputnik, Covaxin, Sinopharm and Sinovac.
The changes in menstrual cycle length did not differ between the vaccines.
The researchers used data from a fertility tracking app called Natural Cycles. Women provided the app with information about their temperature and menstrual cycle length. Users of the app can select an option to provide their data for research purposes without any personal data.
Researchers had published preliminary results in January suggesting a link between the Covid vaccination and longer menstrual cycle length, and the study published this week confirmed the link. The NIH provided five research institutions with $1.67 million to study the problem.
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