Self-Reported Data Link Luteal-Phase COVID Vaccination to Earlier Breakthrough Infections in Small Study

Women vaccinated against COVID-19 during the luteal phase of their menstrual cycle reported breakthrough infections 35 days earlier on average than those vaccinated during the follicular phase, according to an analysis of self-reported data. Researchers examined records from 1,474 women in the US, UK, Canada and Australia who used the Clue period-tracking app in 2021.

Of these, 82 reported catching COVID-19 after vaccination, most with Pfizer or Moderna shots.

The luteal phase occurs when progesterone levels are elevated and the egg travels to the uterus, while the follicular phase features higher estrogen and maturation of an egg-containing follicle. A 2022 meta-analysis cited in the study showed lower levels of several immune-related proteins, including antibodies, during the luteal phase.

Poppy Cooper at the London School of Hygiene & Tropical Medicine said the menstrual cycle has long been treated as background noise in health research and that the work reflects a shift toward recognizing its broader effects.

The study, published in npj Women’s Health, did not confirm infections with PCR tests and did not account for the menses or ovulation stages. Some researchers work for Clue, and the observational design does not establish causation. Julia Craggs, a doctor specializing in women’s health, said the findings raise awareness that sex hormones influence physiology and that the menstrual cycle should be treated as a variable that matters rather than one to control away.

Cooper is now investigating whether hormonal contraceptives affect vaccine outcomes. She stressed that vaccines remain important regardless of cycle timing.