Many women have reported changes to their periods after a COVID-19 vaccination.
But a new observational study suggests that changes are short-lived and small, compared with natural cycle variation.
The study drew on data from 3959 Americans who logged at least six consecutive cycles on a menstrual cycle tracking app. Of these, 2403 were vaccinated and the remainder acted as a control group.
After accounting for other factors, the first dose of vaccine had no effect on timing of the subsequent period, while the second dose was associated with a delay of 0.45 days. Most affected were the 358 individuals who received both doses of the vaccine in the same cycle, experiencing a 2.32 day delay to their next period.
In all groups, cycle lengths returned to normal by two cycles after vaccination.