JAMA Netw Open

Flu vaccine linked to temporary menstrual cycle changes

April 30, 2025

Influenza vaccination, alone or with COVID-19 vaccination, is associated with small but temporary changes in menstrual cycle length.

Study details: This global retrospective cohort study assessed changes in menstrual cycle length following influenza vaccination, with or without concurrent receipt of a COVID-19 vaccine. Data were collected from April 25, 2023, to February 27, 2024, among international English-speaking users of a digital birth control application. Participants included 1,501 individuals aged 18 to 45 years, not using hormonal contraception, and with regular cycle lengths.

Results: Individuals vaccinated for influenza alone experienced an adjusted mean increase of 0.40 days in menstrual cycle length, while those vaccinated concurrently for influenza and COVID-19 experienced a mean increase of 0.49 days. Both groups returned to their prevaccination cycle lengths in the postvaccination cycle. Menstrual cycle changes occurred in the follicular phase but not the luteal phase.

Source:

Boniface ER, et al. (2025, April 1). JAMA Netw Open. Menstrual Cycle Length Changes Following Vaccination Against Influenza Alone or With COVID-19. https://pubmed.ncbi.nlm.nih.gov/40299380/