NIH funded study shows small, but clinically insignificant changes to menstrual cycles post vaccination

by mcardinal

Lauren Dempsey, MS in Biomedicine and Law, RN, FISM News 


A new study has revealed that COVID-19 vaccinations can cause slight changes to a woman’s menstrual cycle, a side-effect that had been a cause of concern for many women throughout the pandemic and one many worried could also affect fertility.

The results from a National Institutes of Health funded study were published early this month in the “Obstetrics & Gynecology Journal,” the official publication of the American College of Obstetricians and Gynecologists (ACOG). The study, which was awarded to five institutions, was granted $1.67 million to investigate changes in women’s menstrual cycles post COVID-19 vaccination. The purpose of this research was to determine if the changes that thousands of women reported were coincidental or if there was an association to the vaccines. 

In the initial COVID-19 vaccine clinical trials researchers did not collect data on menstrual cycle changes or any information related to participants’ reproductive systems. This is because the trials were focused on assessing severe adverse reactions and safety issues. For women in particular, the lack of research in this area was concerning as menstrual cycle characteristics can be an important indicator of health and fertility.  

The study analyzed data from a fertility-awareness application called “Natural Cycles” that gathered information from 3,959 individuals between the ages 18-45 years old with normal cycle lengths. There was a total of 2,403 vaccinated participants and 1,556 unvaccinated participants in the study.

Researchers gathered data from October 2020 to September 2021, with the COVID-19 vaccines administered between December 2020 and July 2021. This study did not include post-menopausal women, who have also reported irregular bleeding post-vaccination, and also excluded individuals who received the Oxford AstraZeneca vaccine. 

Researchers then evaluated data “for three consecutive cycles before the first vaccine dose, followed by vaccine-dose cycles (cycles 4–6) or, if unvaccinated, six cycles over a similar time period.”

 The results of the study showed that small menstrual cycle changes were, in fact, associated with COVID-19 vaccination. “Unvaccinated individuals saw not significant change” to menstrual cycle length when compared to at least three baseline cycles. In individuals that had been vaccinated there was “a less than 1-day change in cycle length for both vaccine-dose cycles compared with pre-vaccine cycles.”

The study authors reported that after they evaluated 23,754 total menstrual cycles there was “statistically significant differences” between the vaccinated and unvaccinated groups. It was noted that these differences were less than 1 day and was “below the reportable difference in the menstrual cycle tracking application,” therefore it was not considered “clinically significant.”

Data from the study, however, also revealed that a subset of vaccinated individuals who received both doses in a single cycle experienced a 2-day increase in cycle length, and “approximately 10% of these individuals experienced a clinically notable change in cycle length of 8 days or more.” This change, however, notably decreased within two postvaccine cycles.

The study cites the International Federation of Gynecology and Obstetrics, which classifies “a variation in cycle length as normal if less than 8 days.” It also notes that menstrual cycles can be affected by stress, which may result in a skipped cycle or changes in cycle length, this normal occurrence may be perceived as concerning, especially in conjunction with a new exposure such as COVID-19 vaccination.” 

The study authors do acknowledge that while this study indicates no clinically significant cycle changes it does not address other questions that remain such as “unscheduled bleeding, and changes in the quality and quantity of menstrual bleeding.”