A new study using menstrual tracking data finds women receiving a COVID-19 vaccination during the follicular phase were more likely to self-report side effects and had a longer time to subsequent infection than those vaccinated during the luteal phase.
The research, led by researchers at London School of Hygiene & Tropical Medicine and the University of Montpellier used self-reported data from menstrual tracking app Clue. Published in npj Women's Health, the study is the first to investigate whether the timing of COVID-19 vaccination within the menstrual cycle is associated with vaccine outcomes. The findings suggest that the menstrual cycle phase may influence vaccine responses and underscore the importance of considering the menstrual cycle in future vaccine and immunology research.
The study found that people vaccinated during the follicular phase of their cycle, when oestrogen is dominant, were more likely to report vaccine side effects than those vaccinated during the luteal phase, when progesterone is dominant. They also appeared to go longer before a subsequent COVID-19 infection, though this finding is still exploratory.
Vaccination during the follicular phase was associated with 35% higher odds of reporting any vaccine side effect, compared with vaccination during the luteal phase (after ovulation). The association was consistent after multiple sensitivity analyses, including excluding participants vaccinated around menstruation, suggesting the findings were not simply explained by premenstrual or menstrual symptoms. The researchers found no evidence that follicular-phase vaccination was associated with more severe side effects or a greater number of side effects.
Participants vaccinated during the follicular phase also went a median of 35 days longer before a subsequent COVID-19 infection (200 days vs 164 days), though with only 82 infections recorded across the sample, the researchers emphasise that this result is exploratory and hypothesis-generating rather than evidence that vaccination timing improves protection.
The study was led by Poppy Cooper at the London School of Hygiene & Tropical Medicine (LSHTM) and Alexandra Alvergne at the University of Montpellier, in collaboration with researchers from Oregon Health & Science University (OHSU), and Clue.
The findings add to a growing body of evidence that reproductive hormones influence immune function. Oestrogen is generally associated with heightened immune activity, while progesterone tends to have a dampening effect. Previous research has shown that women often mount stronger antibody responses than men to vaccines, including for influenza, MMR, and hepatitis, and often report more side effects. This study is among the first to examine whether those biological variations may be associated with differences in vaccine response across the cycle.
- Cooper PA, Boniface ER, Darney BG, et al. Menstrual cycle phase and its association with COVID-19 vaccine outcomes among period tracking app users. npj Womens Health. Published online 06 June 2026. doi:10.1038/s44294-026-00150-x