An extensive study funded by the NIH that included participants from North America and Europe found a connection between receiving the COVID-19 vaccine and a temporary lengthening of the menstrual cycle.
The results of a previous 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 number of menstrual days did not change along with the increase (days of bleeding).
The National Institutes of Health provided funding for the new study, which included information from nearly 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 experienced a decrease in the increase during the cycle after vaccination.
Alison Edelman, M.D., M.P.H., of Oregon Health & Science University in Portland served as the study’s principal investigator. It is published in BMJ Medicine.
The director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, Diana Bianchi, M.D., said that these findings “provide additional information for counseling women on what to expect after vaccination” (NICHD). “Post-vaccination changes appear to be minor, within the range of normal variation, and temporary.”
The study, which was part of a $1.67 million grant given to five institutions to investigate potential connections between COVID-19 vaccination and menstrual changes, was funded by the NICHD and the NIH’s Office of Research on Women’s Health.
Less than an 8 day shift in cycle length is regarded as being within the range of normal variation. The study’s authors noted that although minor menstrual changes may not be significant to medical professionals and researchers, perceived changes in a bodily function associated with fertility may be alarming to those experiencing them and may contribute to vaccine hesitancy.
De-identified data from the fertility tracking app Natural Cycles was examined by researchers. The app’s users gave it data about their body temperatures and menstrual cycles. Additionally, they could have approved the use of their de-identified data for research.
The COVID-19 vaccine’s widespread distribution allowed the study’s authors to go beyond their initial investigation of Americans. Data from participants from all over the world were included in the study, but the majority came from the United Kingdom (32%), the United States and Canada (29%) and Europe (34%).
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Participants also received COVID-19 vaccines made from engineered 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 vaccination and at least one cycle following were examined by the researchers. For unvaccinated participants, data from at least four successive cycles were analyzed over a comparable time frame.
In each vaccination cycle, those who received the vaccine on average saw 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 participants who were not vaccinated, cycle length increased only slightly after vaccination—by.02 days for those who received one dose per cycle and by.85 days for those who received two doses. The type of vaccine received had no effect on changes in cycle length.
In total, 1,342 participants—including 6.2% of those who had received vaccinations and 5.0% of those who hadn’t—experienced a change in cycle length of eight or more days. Younger women and those with longer prevaccination cycles were more likely to experience the increase.
Future research on other effects of vaccination-related changes to menstrual cycles, such as unexpected vaginal bleeding, menstrual flow, and pain, was encouraged by the authors. They also demanded research to identify the physical causes of potential causes of such changes.