Menstrual cycle timing may change COVID vaccine protection

A new analysis of app-tracked vaccination data suggests women vaccinated during the luteal phase may get COVID-19 sooner than those vaccinated during the follicular phase—adding momentum to a long-missed idea: the menstrual cycle shouldn’t be treated as “backg

For years, the menstrual cycle has hovered at the edge of medical studies—often treated as something to control for, not something to understand. But a growing body of evidence is pulling it into the center of how researchers think about the immune system.

Now, new research points to a timing effect. It suggests that the degree of protection women get from vaccines may vary depending on where they are in their menstrual cycle when they receive them, with the latest work focusing on how soon after COVID-19 vaccination women catch the infection.

Poppy Cooper, at the London School of Hygiene & Tropical Medicine, frames the shift bluntly. “For too long, the menstrual cycle has been treated as background noise in health research,” she says. “This work is part of a growing shift in that thinking and a recognition that the menstrual cycle has impacts beyond reproductive health.”.

Her team’s interest also follows a moment that landed in public consciousness early in the vaccine rollout. Soon after COVID-19 vaccines were introduced in 2021, reports emerged from women who said their periods became heavier and began earlier. A later study found that if those changes occurred, they tended to be small and temporary.

Cooper and colleagues have taken that conversation in the opposite direction—asking not what vaccines might do to periods. but what periods and their hormones might do to vaccine outcomes. Their analysis used data from 1. 474 women in the US. the UK. Canada. and Australia who tracked their periods using the Clue app in 2021. These women also self-reported their vaccine outcomes through an in-app survey.

Out of that group, 82 women reported catching COVID-19 after vaccination, mostly after receiving Pfizer or Moderna. When researchers compared timing within the menstrual cycle. the difference was stark in the numbers they reported: breakthrough infections happened 35 days earlier among those vaccinated during the luteal phase—when progesterone is high and the egg makes its journey to the uterus—than among those vaccinated during the follicular phase. when oestrogen is high. the uterus lining thickens. and an egg-containing follicle matures within one of the ovaries.

The study points toward a biological explanation. Hormones fluctuate across the cycle, and those fluctuations may influence how immune cells respond to vaccines. A 2022 meta-analysis found that many immune-related proteins, including antibodies, are lower in the luteal phase.

Julia Craggs, a doctor who specialises in women’s health, puts progesterone’s role in plain terms. “Progesterone’s job, biologically, is to prepare the body to tolerate a potential pregnancy,” she says. “One way it does that is by turning down some of the more aggressive parts of the immune system. like T cell activity. That isn’t necessarily what you want at the precise moment you are trying to train the immune system to recognise a vaccine antigen [a substance that triggers the immune system to mount a response].”.

Still, the researchers are clear that the evidence has limitations. The number of COVID-19 infections reported was relatively small, and the infections weren’t confirmed by a PCR test. There were also gaps in the way the cycle was accounted for: the study didn’t include the other two stages of the menstrual cycle—menses (a period) and ovulation. And because the research is observational, it can’t establish that the menstrual cycle directly causes differences in infection timing.

Even with those caveats. Craggs says the work matters because it changes what doctors and researchers treat as “noise” versus signal. “This study treats the menstrual cycle as a variable that matters rather than a confounding variable to simply control away. ” she says. “We may be sitting on a significant source of unexplained variation in how women respond to treatment and we have simply never looked in that direction.”.

Cooper. for her part. stresses that vaccines remain important for protecting against infections and ill health. no matter where someone falls in their cycle. She also wants the question to become routine in medical research rather than an exception sparked by curiosity. “I’d love to see the same question asked across other vaccines and medical interventions,” she says. Cooper is now investigating whether hormonal contraceptives affect vaccine outcomes.

If the menstrual cycle can indeed shift how quickly immunity is translated into real-world protection. it’s a reminder that “one size fits all” may never have been true for biology. The next step is making sure the science catches up—systematically. not sporadically—before guidance has to be built around unanswered questions.

menstrual cycle vaccine effectiveness COVID-19 progesterone luteal phase follicular phase immune response Clue app Pfizer Moderna women’s health hormonal contraceptives