Participants in a sizable NIH-funded study came from both North America and Europe.
The results of a prior American study that connected COVID-19 vaccination with an average increase in menstrual cycle length of less than one day were supported by a sizable international study.
The National Institutes of Health provided funding for the new study, which included information from over 20,000 individuals from Canada, the United Kingdom, the United States, Europe, and other countries who received any one of nine different vaccines.
Most study participants had a decrease in the rise during the cycle after vaccination. The number of menstrual days did not vary along with the increase (days of bleeding).
“These findings provide additional information for counseling women on what to expect after vaccination,” said Diana Bianchi, M.D., director of NIH’s 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.”
Alison Edelman, M.D., M.P.H., of Oregon Health & Science University in Portland served as the study’s lead investigator. It is published in BMJ Medicine. The study, which was part of a $1.67 million grant given to five universities to investigate potential connections between COVID-19 immunisation and menstrual abnormalities, was supported by the NICHD and the NIH’s Office of Research on Women’s Health.
Less than an eight-day shift in cycle length is seen as being within the range of typical variation. The study’s authors noted that although minor menstrual alterations may not be significant to medical professionals and researchers, perceived changes in a bodily function associated with fertility may be frightening to persons experiencing them and may contribute to vaccine hesitation.
De-identified data from the fertility tracking app Natural Cycles was examined by researchers. The app’s users gave it data on their body temperatures and menstrual cycles. Additionally, they could have approved the use of their de-identified data for study. The COVID-19 vaccine’s widespread distribution allowed the study’s authors to go beyond their first investigation of Americans. Data from participants from all around the world were included in the study, however the majority were from the United Kingdom (32%), the United States and Canada (29%) and Europe (34%). Participants also received COVID-19 vaccines produced from modified viruses (AstraZeneca, Covishield, Janssen/Johnson & Johnson, and Sputnik), inactivated viruses (Pfizer-BioNTech and Moderna), and messenger RNA vaccines (Pfizer-BioNTech and Moderna) (Covaxin, Sinopharm, and Sinovac).
19,622 people participated in total. 14,936 of them received vaccinations, while 4,686 did not. Data from at least three consecutive cycles prior to immunisation and at least one cycle following were examined by the researchers. For unvaccinated subjects, data from at least four successive cycles were evaluated during a comparable time frame.
In each vaccination cycle, those who received the vaccine on average had their length of stay increase by less than one day: by.71 days after the first dose and by.56 days after the second. A participant’s cycle length increased by 3.91 days when both doses were administered in the same cycle.
In comparison to those who were not vaccinated, cycle length increased just slightly after vaccination—by.02 days for those who received one dose each cycle and by.85 days for those who received two doses. The type of vaccine received had no effect on changes in cycle length. 1,342 participants out of the total had a change in cycle duration of eight or more days, including 6.2% of the vaccinated and 5.0% of the unvaccinated. Younger women and those with lengthier prevaccination cycles were more likely to experience the rise.
Future research on other effects of vaccination-related alterations to menstrual cycles, such as unexpected vaginal bleeding, menstrual flow, and pain, was encouraged by the authors. They also recommended research to identify the physical causes of potential causes of such alterations.
Parvathy Sukumaran 
