New research in npj Women’s Health reports that timing COVID-19 vaccination within the menstrual cycle is associated with differences in reported side effects. Using menstrual-cycle data from a period tracking app and a COVID-19 vaccination survey of 1,474 women, researchers mapped vaccine dates to cycle phases. The study defined the luteal phase as the 14 days before menstruation and the remaining days as the follicular phase. Vaccination during the follicular phase was associated with higher odds of side effects—such as injection site pain, fatigue, and aches—compared with luteal-phase vaccination. The analysis also indicated a longer median time to infection among those vaccinated in the follicular phase (with only 82 reported infection cases), limiting firm conclusions. The authors tied the pattern to hormone differences across phases, including rising estrogen levels in the follicular phase and progesterone’s potential immunomodulatory effects during the luteal phase.