Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone or With COVID-19

Multiple studies have identified an association between COVID-19 vaccination and menstrual disturbances. Data on whether menstrual health is impacted by other vaccines are needed to counsel individuals about what to expect and to address vaccine hesitancy.

To assess the association of changes in length of the menstrual cycle with influenza vaccination, with or without concurrent receipt of a COVID-19 vaccine.

This global retrospective cohort study prospectively collected menstrual cycle data from April 25, 2023, to February 27, 2024 (4-5 cycles per individual), among international English-speaking users of a digital birth control application. Participants included individuals aged 18 to 45 years, not using hormonal contraception, and with average cycle lengths of 24 to 38 days in 3 consecutive cycles before receipt of vaccines.

Seasonal influenza vaccination with or without concurrent receipt of COVID-19 vaccine.

The primary outcome consisted of adjusted mean within-individual changes of menstrual cycle length assessed by vaccination group. Secondary analysis evaluated the phase of menstrual cycle at time of vaccination.

A total of 1501 individuals met the inclusion criteria, of whom 791 were vaccinated for influenza only and 710 were concurrently vaccinated for influenza and COVID-19. By race and ethnicity, 1 participant (0.1%) was American Indian or Alaska Native; 10 (0.7%), Asian; 3 (0.2%), Black; 15 (1.0%), Hispanic or Latina; 1 (0.1%), Middle Eastern or North African; 368 (24.5%), White; and 19 (1.3%), other; and 1084 (72.2%), missing. Most of the cohort was younger than 35 years (1230 [82.0%]), had at least a college degree (1122 [74.8%]), and was located in the US or Canada (938 [62.5%]). Individuals vaccinated for influenza alone experienced an adjusted mean increase of 0.40 (95% CI, 0.08-0.72) days, while those vaccinated concurrently for influenza and COVID-19 experienced a mean increase of 0.49 (95% CI, 0.16-0.83) days (P = .69 for difference between vaccine groups). A total of 37 individuals (4.7%) experienced a change in cycle length of at least 8 days with influenza vaccine only and 42 (5.9%) with concurrent receipt of both vaccines (P = .28). In the postvaccination cycle, both vaccination groups returned to their prevaccination cycle lengths. Menstrual cycle changes occurred with vaccination in the follicular phase but not the luteal phase.

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.