COVID-19 vaccination in follicular phase tied to vaccine-related menstrual changes
Click Here to Manage Email Alerts
Key takeaways:
- Women who received COVID-19 vaccination in the follicular phase had longer menstrual cycles vs. before vaccination.
- Vaccination in the luteal phase or no vaccination were not linked to menstrual cycle changes.
SAN FRANCISCO — Receiving the COVID-19 vaccine in the first vs. last half of the menstrual cycle was associated with vaccine-related cycle length changes, according to a presentation at the ACOG Annual Clinical & Scientific Meeting.
“During the initial vaccine rollout, there were many reports of menstrual changes following vaccination on social media and on event reporting platforms in the U.S. and around the world,” Emily R. Boniface, MPH, biostatistician and doctoral student at Oregon Health & Science University at the School of Public Health, said during the presentation. “Reports and research looking at menstrual cycle changes following vaccination date back as far as 1913, and in the 1980s, concerns about vaccination across the nation led to vaccine hesitancy and decreased vaccine uptake.”
Boniface and colleagues conducted a retrospective cohort analysis using menstrual tracking data from 19,497 reproductive-aged female users of the Natural Cycles app from 2021 to 2022. Researchers identified first COVID-19 vaccine dose timing and evaluated changes in menstrual cycle length in the second vaccination cycle and, if a clinically significant change of 8 days or more in cycle length occurred, in either vaccination cycle. All women had an average cycle length for 3 months prior to vaccination.
Primary outcome was change in menstrual cycle length from the average length of the three cycles before first COVID-19 vaccination.
Overall, 9,279 women received their first COVID-19 vaccination in the follicular phase of the menstrual cycle, 5,532 received the vaccine in the luteal phase, and 4,686 women were unvaccinated. Most women (80.1%) were younger than 35 years and, among vaccinated women, 63.8% received an mRNA vaccine.
Women who received the COVID-19 vaccine in the follicular phase had an average of 1 day longer menstrual cycle length with the first (adjusted change = 1 day; 98.75% CI, 0.88-1.13) or second (adjusted change = 1.11 days; 98.75% CI, 0.93-1.29) dose compared with the average cycle before vaccination. Women who received the COVID-19 vaccine in the luteal phase and unvaccinated women had no changes in menstrual cycle length compared with the average cycle length before vaccination.
In addition, women who were vaccinated in the follicular phase had a higher likelihood of experiencing a clinically significant change of 8 days or more in menstrual cycle length with the first vaccine dose compared with women vaccinated in the luteal phase or unvaccinated women (6.8% vs. 3.3% and 5%, respectively; P < .001). Results were similar for second vaccine doses with a higher likelihood of experiencing a clinically significant change of 8 days or more in menstrual cycle length for women vaccinated in the follicular phase vs. the luteal phase or unvaccinated women (6.8% vs. 4.7% and 5%, respectively; P = .001).
After a new medical intervention, any unanticipated change in a routine bodily function tied to fertility may increase vaccine hesitancy and, therefore, lead to long-lasting impacts, the researchers wrote, which is why it is important to understand why changes are occurring.
“We find that COVID-19 vaccination-related cycle length changes are associated with receiving vaccination in the first half of the cycle,” Boniface said. “We hope that these findings can validate the public reports of menstrual changes while also reassuring the public that these are small, temporary changes and they are not clinically significant for the vast majority of individuals.”