On Sept. 27, 2022, the National Institutes of Health announced that a large international study had confirmed the findings of a previous U.S. study that linked COVID-19 vaccination with an average increase in menstrual cycle length of less than one day. The increase was not associated with any change in the number of days of menses (days of bleeding).

Funded by the National Institutes of Health, the new study included data from nearly 20,000 people from Canada, the United Kingdom, the United States, Europe and other parts of the world who received any of nine different vaccines. For most study participants, the increase resolved in the cycle following vaccination.