However, the longer menstrual cycle, typically about a month long, didn't necessarily increase the number of days of bleeding, according to health agency.
Women who received both vaccine doses in a single menstrual period saw their cycle increase by 3.91 days.
After the vaccination series was complete, cycle length had largely returned to normal for women who received one dose per menstrual cycle and by about 20 hours for those who received both doses in a single cycle.