COVID-19 vaccine–related cycle length changes linked to receipt of vaccine during follicular phase

Receipt of the COVID-19 vaccine during the follicular phase of the menstrual cycle leads to changes in cycle length, as reported in a study.

For the study, researchers examined prospectively collected menstrual cycle tracking data from 19,497 reproductive-aged users of the application “Natural Cycles.” They also identified the timing of vaccine delivery (ie, during the follicular or luteal phase).

The primary endpoint was the adjusted within-individual change in cycle length (in days) from the average of the three menstrual cycles before the first vaccination cycle. This was compared between vaccinated and unvaccinated participants, with the unvaccinated control group being assigned a notional vaccine date. Cycle length changes in the second vaccination cycle and whether a clinically significant change in cycle length (8 days or more) occurred in either cycle were also evaluated.

In the overall cohort, 80.1 percent of participants were younger than age 35 years, and 31.7 percent were from the United Kingdom. In the vaccinated group, 63.8 percent of participants received an mRNA vaccine.

The adjusted cycle length with a first or second dose of COVID-19 vaccine was longer by an average of 1 day for participants vaccinated in the follicular phase when compared with the prevaccination cycle length average (first dose: 1.00 day, 98.75 percent confidence interval [CI], 0.88–1.13; second dose: 1.11 days, 98.75 percent CI, 0.93–1.29).

Participants vaccinated in the luteal phase did not experience any change in cycle length (first dose: −0.09 days, 98.75 percent CI, −0.26 to 0.07; second dose: 0.06 days, 98.75 percent CI, −0.16 to 0.29), as did those in the unvaccinated control group (notional first dose: 0.08 days, 98.75 percent CI, −0.10 to 0.27; second dose: 0.17 days, 98.75 percent CI, −0.04 to 0.38).

Additionally, participants vaccinated during the follicular phase were more likely to experience a clinically significant change in cycle length (8 days or more; first dose: 6.8 percent) compared with those vaccinated in the luteal phase or those who were unvaccinated (3.3 percent and 5.0 percent, respectively; p<0.001).