New Research using Clue Data Suggests Menstrual Cycle Timing May Influence Vaccine Side Effects

A new study using Clue's menstrual tracking data finds women vaccinated 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

BERLIN, July 29, 2026 (GLOBE NEWSWIRE) -- Clue, the #1 women-led period and cycle tracker, today announced new research led by researchers at London School of Hygiene & Tropical Medicine and the University of Montpellier using menstrual tracking data from 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 estrogen 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.

Key study findings:

• Vaccination during the follicular phase (the phase of the cycle before ovulation when estrogen is dominant) was associated with 35% higher odds of reporting any vaccine side effect, compared with vaccination during the luteal phase (after ovulation, when progesterone is dominant).
  • 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 emphasize 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. Estrogen 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.

"We know that immune function fluctuates across the menstrual cycle, yet it's still rarely considered in mainstream health research," said Amanda Shea, PhD, fractional Chief Science Officer at Clue. "To truly understand women's health, we need to stop treating the menstrual cycle as background noise and start recognizing it as a fundamental part of human biology. Incorporating cycle data into research is essential to building the evidence needed for more personalized and effective care."

Research has been central to Clue's mission since the company's founding. With more than 250 million tracked cycles and over 30 billion data points from consenting app users, Clue holds one of the largest longitudinal menstrual health datasets in the world, and partners with researchers at institutions including MIT, Oxford, Columbia, and UC Berkeley to advance the science of menstrual and reproductive health. Women's health has long been understudied relative to its impact, and closing that gap is central to why Clue exists. This study was possible because Clue users chose to contribute their tracked cycle data to research, data that allowed scientists to investigate a question about vaccine response that had never before been possible to answer.

"Every time the Clue community tracks their cycle, they're helping answer a question about their own body that medicine has too often ignored," said Rhiannon White, CEO of Clue. "This study is a small example of something bigger: when women are given the tools to understand themselves, and the research catches up to meet them, real change becomes possible."

More information about the study can be found here.

Methodology:

Researchers surveyed Clue users on the timing of their first COVID-19 vaccine dose, any side effects experienced, and the details of any subsequent COVID-19 infections. These responses were matched against each participant's own previously tracked cycle data to determine the menstrual cycle phase at the time of vaccination, rather than relying on recalled estimates. Of the 1,474 users included in the analysis, 760 were vaccinated during the follicular phase and 714 during the luteal phase.

The researchers emphasize that the findings should not be used to time vaccination around cycle phase. Receiving a vaccine when a dose is available remains far more important than the timing within a cycle, and as the first study of its kind, further evidence is needed before any recommendations could be made. The findings do, however, support the case for including menstrual cycle phase as an important variable in future vaccine and immunology research.

About Clue

Clue is the #1 women-led period and cycle tracker, used by more than 100 million people across 190 countries who have opted in to track more than 250 million cycles. Made by BioWink GmbH in Berlin, Clue partners with research institutions including MIT, Oxford, and Columbia to advance the science of menstrual and reproductive health.

Media Contact: press@helloclue.com

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