A study in npj Women’s Health reported that menstrual cycle phase may influence COVID-19 vaccine side-effect reporting. Using menstrual cycle data from a period tracking app matched to vaccination dates from a survey of 1,474 women, researchers compared outcomes between the follicular phase and the luteal phase. The analysis found those vaccinated in the follicular phase had higher odds of reporting side effects, including injection site pain, fatigue, and aches and pains, while also showing a longer median time to infection in the available dataset. The authors noted infection event counts were small, limiting firm conclusions for infection outcomes.