One of the first studies to track whether COVID-19 vaccination might affect women’s periods found a small and temporary change. Research published Wednesday tracked nearly 4,000 U.S. women through six menstrual cycles and on average, the next period after a shot started about a day later than usual. But there was no change in the number of days of menstrual bleeding after COVID-19 vaccination. … Some women have reported irregular periods or other menstrual changes after their shots.