The timing of COVID-19 vaccination within the menstrual cycle may be associated with self-reported side effects, according to research published in npj Women’s Health. Researchers analysed data from Clue, a period-tracking application, matching logged menstrual data with participants’ self-reported vaccine experiences. The final analytical sample included 1,474 individuals aged 18 to 44 years with regular cycle lengths, excluding those using hormonal contraceptives or with recent pregnancies.
Participants were classified based on their estimated menstrual phase at vaccination: follicular phase (oestrogen-dominant, n=760) or luteal phase (progesterone-dominant, n=714). Among those vaccinated during the follicular phase, 75.9% reported side effects compared with 70.3% in the luteal phase.
After adjusting for age, body mass index, smoking status, and pre-existing conditions, participants vaccinated during the follicular phase had 35% higher odds of reporting any side effects than those vaccinated during the luteal phase. The association was particularly pronounced in younger participants aged 18 to 24, who showed more than twice the odds of reporting side effects when vaccinated during the follicular phase.
No statistically significant association was found between cycle timing and the severity of side effects or the total number of side effects reported. Exploratory analysis suggested a longer median time to breakthrough infection among those vaccinated during the follicular phase (200 versus 165 days), though this finding was underpowered due to the small number of reported infections.
The researchers acknowledged limitations including reliance on calendar-based phase estimates rather than hormonal biomarkers, self-reported outcomes, and potential recall bias. They emphasised that the findings are preliminary and serve as a basis for future research examining whether menstrual cycle timing affects vaccine responses.
Source: News Medical / npj Women’s Health (2026)
