WEDNESDAY, Sept. 28, 2022 (HealthDay News) -- Women may experience a small, but temporary, delay in their menstrual cycle after receiving a COVID shot, a new study finds.
The delay was, on average, less than one day and, for most women, it resolved after the first cycle post-vaccination, according to this research funded by the U.S. National Institutes of Health. The results mirrored the findings of an earlier U.S. study.
"These findings provide additional information for counseling women on what to expect after vaccination," said Dr. Diana Bianchi, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD).
"Changes following vaccination appear to be small, within the normal range of variation, and temporary," she said in an NIH news release.
The study included 20,000 women in the United States, Canada, United Kingdom, Europe and other parts of the world.
Any cycle length change of less than eight days is considered in the normal range. While considered not meaningful from a health care perspective, perceived changes in a bodily function linked to fertility may be alarming to those experiencing it and could contribute to vaccine hesitancy, the authors noted.
To study the issue, researchers used the data of 19,622 women from Natural Cycles, a fertility tracking app. Data included information on women's temperatures and menstrual cycles.
Study participants had received a range of vaccines, depending on their country, including mRNA COVID-19 vaccines from Pfizer and Moderna, engineered virus vaccines from AstraZeneca, Covishield, Johnson & Johnson and Sputnik, and inactivated virus vaccines from Covaxin, Sinopharm and Sinovac.
In all, 14,936 study participants were vaccinated and 4,686 were not. Researchers used data from at least one cycle after vaccination, as well as at least four before vaccination. The team also analyzed at least four consecutive cycles of data from a similar time interval for those who were not vaccinated.
Vaccinated people on average experienced a .71 day increase in time between cycles after the first dose and a .56 increase after the second dose. Those who had received both of their doses in a single cycle had a 3.91 day increase in cycle length.
Compared to unvaccinated participants, after vaccination the cycle length had increased by only .02 days for individuals who received one dose per cycle and .85 days for individuals who received two doses in one cycle.
Vaccine type didn't cause additional variations.
About 6.2% of vaccinated individuals and 5% of unvaccinated individuals had a change in cycle length of eight or more days. Researchers found that women who were younger and who had longer menstrual cycles before vaccination were more likely to experience the increase.
Future studies should be done on other aspects of vaccination-linked changes to menstrual cycles, such as unexpected vaginal bleeding and menstrual flow and pain, the authors said. They also called for studies to determine why these changes might occur.
The research was done using part of $1.67 million awarded to five institutions for the purpose of exploring links between COVID vaccination and menstruation. Principal investigator was Dr. Alison Edelman, from Oregon Health & Science University in Portland.
The findings were published in the BMJ Medicine.
The U.S. Centers for Disease Control and Prevention has more on COVID-19 vaccines.
SOURCE: U.S. National Institutes of Health, news release, Sept. 27, 2022