For a study, researchers sought to see if coronavirus disease 2019 (COVID-19) immunization caused changes in a cycle or menstrual duration when compared to an unvaccinated control group. Using the “Natural Cycles” program, they evaluated prospectively collected menstrual cycle data. They enrolled 18–45-year-olds in the United States with typical cycle durations (24–38 days) for 3 consecutive cycles before the first vaccination dosage, followed by vaccine-dose cycles (cycles 4–6) or, if unvaccinated, 6 cycles over a comparable time span. The mean within-individual change in the cycle and menstrual duration was computed (three pre-vaccine cycles vs first- and second-dose cycles in the vaccinated cohort, and the first three cycles vs cycles four and five in the unvaccinated cohort). The adjusted difference in the cycle and menstrual duration change between the vaccinated and unvaccinated cohorts was estimated using mixed-effects models.
They enlisted the help of 3,959 people (vaccinated 2,403; unvaccinated 1,556). The Pfizer-BioNTech vaccine was administered to the majority of the immunized population (55%) (Moderna 35%, Johnson & Johnson/Janssen 7%). Overall, the COVID-19 vaccine was associated with a less than 1-day increase in cycle length for both vaccine-dose cycles when compared to pre-vaccine cycles (first dose 0.71 day-increase, 98.75% CI 0.47–0.94; second dose 0.91, 98.75% CI 0.63–1.19); unvaccinated individuals saw no significant difference when compared to three baseline cycles (cycle four 0.07, 98.75% CI 0.22 to 0.35; cycle five 0.12, 98.75% CI −0.15 to 0.39). The difference in cycle length between the vaccinated and unvaccinated groups was less than 1 day in adjusted models for both doses (difference in change: first dose 0.64 days, 98.75% CI 0.27–1.01; second dose 0.79 days, 98.75% CI 0.40–1.18). Vaccination had no effect on menstrual cycle duration. The immunization against Coronavirus disease 2019 (COVID-19) was connected with a slight change in cycle duration but not in menstrual length.
