A new study indicates that the phase of a woman's menstrual cycle at the time of COVID-19 vaccination may influence how soon breakthrough infections occur. Researchers found earlier infections among those vaccinated during the luteal phase compared with the follicular phase.
The analysis examined data from 1,474 women in the US, UK, Canada and Australia who used the Clue period-tracking app in 2021 and reported vaccine outcomes. Eighty-two women experienced breakthrough infections, mostly after Pfizer or Moderna shots.
Breakthrough infections occurred 35 days earlier on average among women vaccinated in the luteal phase, when progesterone levels are high, than in the follicular phase, when oestrogen dominates. Poppy Cooper of the London School of Hygiene & Tropical Medicine noted that the menstrual cycle has long been overlooked in health research.
Julia Craggs, a women's health doctor, explained that progesterone can dampen certain immune responses, which may affect how the body builds protection from a vaccine. The study did not confirm infections by PCR test and did not examine all cycle stages.
Cooper emphasised that vaccines remain important regardless of cycle timing but called for more research into how hormones influence medical interventions.
