Researchers using period tracking app data and a vaccination survey of 1,474 women reported that menstrual-cycle phase at vaccination correlated with different COVID-19 vaccine outcomes. Published in npj Women’s Health, the study compared side effects in women vaccinated during the follicular phase versus the luteal phase (defined as the two weeks preceding menstruation versus the remainder of the cycle). The team found women vaccinated during the follicular phase reported 35% higher odds of side effects such as injection site pain, fatigue, and aches and pains. The analysis also suggested a longer median time to infection after follicular-phase vaccination, though the authors cautioned that only 82 infection cases limited statistical power. The study points to hormonal shifts—estrogen rising in the follicular phase versus progesterone during the luteal phase—as a potential explanation, while emphasizing the need for further investigation due to the small infection subgroup.