How Influenza and COVID-19 Vaccinations May Impact Menstrual Cycles, finds study
Recently published study investigates the effects of influenza vaccination, both alone and in conjunction with a COVID-19 vaccine, on menstrual cycle length in individuals aged 18 to 45 not using hormonal contraception. Conducted as a global retrospective cohort study between April 2023 and February 2024, data were collected from users of a digital birth control app, focusing on those with established average cycle lengths of 24 to 38 days prior to vaccination.
Outcome Measures
The primary outcome measured was the adjusted mean changes in menstrual cycle length by vaccination group, while secondary analyses considered the menstrual cycle phase at the time of vaccination. Notably, the study addresses a significant gap in research concerning the impact of various vaccinations on menstrual health, especially amid concerns raised regarding COVID-19 vaccinations previously linked to menstrual disturbances.
Key Findings
Findings indicated that both vaccination groups experienced negligible but statistically significant increases in cycle length during the vaccination cycle—0.40 days for those receiving only the influenza vaccine and 0.49 days for those receiving both vaccines. However, in the post-vaccination cycle, there were no substantial changes compared to prior cycles. Additionally, the prevalence of clinically meaningful changes (defined as an increase of 8 days or more) was slightly higher in the group vaccinated with both vaccines (5.9% vs. 4.7%), but this difference lacked statistical significance.
Sensitivity Analysis
Sensitivity analyses were conducted excluding individuals with certain gynecological disorders and considering other demographic variables to validate findings. Notably, significant increases in cycle length were observed only for individuals vaccinated during the follicular phase of their menstrual cycle, with no changes noted for those vaccinated in the luteal phase.
Conclusions and Implications
The report highlights that while temporary changes in menstrual cycle length may occur in response to vaccination, concurrent administration of influenza and COVID-19 vaccines does not appear to increase the risk of significant menstrual cycle disturbances. This research provides valuable insights, confirming the minimal impact of influenza vaccination on menstrual health and aiming to alleviate concerns that may contribute to vaccine hesitancy. Overall, the findings underscore the need for transparent communication regarding the potential effects of vaccinations, especially in the context of menstrual health, to foster greater public trust in immunization efforts.
Key Points
- A global retrospective cohort study was conducted to evaluate the effects of influenza and COVID-19 vaccinations on menstrual cycle length in individuals aged 18 to 45 who do not use hormonal contraception, with participants having an established average cycle length of 24 to 38 days prior to vaccination.
- The primary outcome analyzed was the adjusted mean changes in menstrual cycle length between different vaccination groups, with secondary analyses focusing on the menstrual cycle phase during vaccination, highlighting a gap in research regarding vaccinations and menstrual health.
- Results indicated that both vaccination groups (influenza only and both influenza and COVID-19 vaccines) experienced slight but statistically significant increases in cycle length during the vaccination cycle—0.40 days for influenza only and 0.49 days for combined vaccines; however, no substantial changes were observed in the post-vaccination cycle.
- The prevalence of clinically meaningful changes (defined as an increase of 8 days or more in cycle length) was higher for those receiving both vaccines (5.9%) compared to the influenza-only group (4.7%), although this difference was not statistically significant.
- Sensitivity analyses revealed that significant increases in cycle length were primarily observed for individuals vaccinated during the follicular phase of their menstrual cycle, with no differences noted for those vaccinated in the luteal phase, suggesting phase-dependent effects of vaccination.
- The study concludes that while there may be temporary increases in menstrual cycle length post-vaccination, administering influenza and COVID-19 vaccines concurrently does not elevate the risk of significant menstrual disturbances, emphasizing the importance of clear communication regarding vaccine effects on menstrual health to improve public trust in vaccination programs.
Reference –
Emily R Boniface et al. (2025). Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone Or With COVID-19. *JAMA Network Open*, 8. https://doi.org/10.1001/jamanetworkopen.2025.7871.
