Does it matter where you are in your menstrual cycle when you get vaccinated? Someone finally decided to find out.
A new study in npj Women’s Health looked at 1,474 women who used the Clue app and got a COVID vaccine. If you were vaccinated during the follicular phase (first half of your cycle, estrogen rising), then there was a 35% higher odds of reporting any side effect, not worse, just more likely to happen in general. Vaccinated during the luteal phase? Fewer reported side effects, but a shorter median time to breakthrough infection.
The infection numbers are small and researchers call that part exploratory, but the real headline is that for decades, the menstrual cycle got treated as noise to control for, not a variable worth studying. This time it was finally the subject.
We’re dropping a full video on the science of period blood at the end of the month on our YT channel and it’s more powerful than you think. Subscribed @tilscience on YT? You’ll want to be.
Read more here: Cooper, P.A., Boniface, E.R., Darney, B.G. et al.Menstrual cycle phase and its association with COVID-19 vaccine outcomes among period tracking app users. npj Womens Health (2026). https://doi.org/10.1038/s44294-026-00150-x
#womenshealth #biology #hormones #hormonehealth #menstrualcycle #menstrualhealth #menstruation #endometriosis #pcos #vaccine #vaccination #covid19