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

Question. Is the influenza vaccine alone or with a COVID-19 vaccine associated with change in menstrual cycle length?

Findings. In this cohort study of 1501 participants, in individuals with regular menstrual cycles, a temporary small but statistically significant increase in menstrual cycle length (<1 day) with receipt of an influenza vaccine, with or without a COVID-19 vaccine, based on vaccination in the follicular phase was found.

Meaning. These findings may help to confirm the utility of vaccination for individuals with concerns about adverse effects of vaccination on menstruation.