TOPLINE:
An influenza vaccine, either given alone or with a COVID-19 vaccine during the follicular phase, was linked to minor changes in the length of the menstrual cycle of less than 1 day, but the length of the cycle bounced back to normal soon after.
METHODOLOGY:
- This global retrospective cohort study assessed temporary changes in the length of the menstrual cycle after influenza vaccination (either alone or with a COVID-19 vaccine), addressing gaps in understanding effects of a non–COVID-19 vaccine.
- Researchers analysed data of 1501 individuals (age, 18-45 years; non-hormonal contraception users) from a birth control app, of whom 791 received an influenza vaccine alone and 710 received influenza and COVID-19 vaccines concurrently.
- The cohort was predominantly younger than 35 years (82.0%), had at least a college degree (74.8%), and was located in the United States or Canada (62.5%). Racial/ethnic data were missing for 72.2% of participants; 24.5% of participants were identified as White. The average length of pre-vaccination cycles was 24-38 days.
- Researchers analysed prospectively tracked cycles (April 2023 to February 2024) and compared each participant’s pre-vaccination cycles at baseline (three cycles) with the vaccination cycle. The adjusted change in the mean length of the cycle was the primary outcome.
- This study compared average changes in the length of the cycle during vaccination betw