Association between COVID-19 vaccines and the menstrual cycle in young Japanese women

INTRODUCTION: Although several studies have investigated the association between coronavirus disease 2019 (COVID-19) vaccines and the menstrual cycle, available data are limited. Therefore, this study investigated the effect of COVID-19 vaccines on the menstrual cycle and the effect of the menstrual cycle phase on the vaccine side effects during vaccine administration in Japan. METHODS: A self-administered questionnaire was used to collect data on the date of vaccination; type of vaccine; type, grade, and duration of the side effects; regularity of menstruation; normal length of the menstrual cycle; and the day one date of menstruation around vaccination. The survey was conducted from October 2021 to March 2022. RESULTS: The difference between the predicted and actual menstrual cycle length was 1.9 ± 3.0, 1.6 ± 2.8 (p = 0.557), and 2.5 ± 3.8 (p = 0.219) days before vaccination and after the first and second dose of the vaccine, respectively. In participants who received vaccinations twice within a single menstrual cycle, this difference was 1.3 ± 3.5 and 3.9 ± 3.3 (p = 0.045) days before and after vaccination, respectively. The grade and proportion of the side effects after the second dose of the vaccine was highest during the menstrual period and lowest during the ovulation period, with a significant effect on headache and chills. CONCLUSION: COVID-19 vaccines tended to prolong the menstrual cycle. The side effects of the COVID-19 vaccine tended to be at a maximum when vaccination occurred during the menstrual period and minimal during the ovulation period.