Influenza and COVID Vaccinations' Effect on Menstrual Cycles

Medscape

06-05-2025

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 between influenza alone and influenza + COVID-19 vaccine groups and looked at post-vaccine and large changes in the length of the cycle (≥ 8 days).
TAKEAWAY:
Influenza vaccination alone led to a 0.40-day increase in the length of the menstrual cycle, whereas concurrent influenza + COVID-19 vaccination led to a 0.49-day increase, with no significant difference observed between two groups ( P = .69).
= .69). A change in the length of the cycle of 8 days or more occurred in 4.7% of influenza-only recipients vs 5.9% of dual-vaccine recipients; however, the difference was not statistically significant ( P = .28).
= .28). Vaccination during the follicular phase led to a significant increase in the length of the cycle (0.82 days for influenza alone and 0.99 days for dual vaccines).
Post-vaccination lengths of the cycles normalised, with mean changes of −0.02 days for influenza alone and 0.14 days for dual vaccines compared with baseline ( P = .46 for the between-group difference).
= .46 for the between-group difference). Individuals vaccinated in the luteal phase showed no significant changes in the length of the cycle (−0.16 days for influenza alone vs −0.14 days for dual vaccines).
IN PRACTICE:
"In this cohort study of individuals with regular menstrual cycles, influenza vaccine given alone or in combination with a COVID-19 vaccine was associated with a small but temporary change in menstrual cycle length. These findings may help clinicians confirm the utility of vaccination for patients with concerns about menstrual adverse effects of vaccination," the authors of the study wrote.
SOURCE:
This study was led by Emily R. Boniface, Department of Obstetrics and Gynecology, Oregon Health & Science University, Portland, Oregon. It was published online on April 29, 2025, in JAMA Network Open.
LIMITATIONS:
The study's high levels of missing demographic data and a mostly young, White, highly educated sample could have limited how widely these results apply. Self-reported vaccine dates could have recall errors. Incomplete cycle data led to exclusions, and unmeasured factors might still influence the findings.
DISCLOSURES:
This study was supported by a grant from the US National Institute of Child Health and Human Development. Several authors reported receiving grants, honoraria, personal fees, and travel fees from various sources.