COVID-19 Vaccines Linked to Minor, Temporary Changes in Menstrual Cycle Length

A large analysis from the Apple Women’s Health Study examined 128,000 menstrual cycles from nearly 10,000 participants. Researchers found that receiving vaccines like Pfizer‑BioNTech COVID‑19 Vaccine, Moderna COVID‑19 Vaccine, or Johnson & Johnson COVID‑19 Vaccine was linked to a very small increase in cycle length (about 0.5–1 day) during the vaccination cycle. Cycles returned to normal within a few months, and the change stayed well within natural variation.