May 5, 2023 – There is no evidence that COVID-19 vaccines cause disruptions in women’s menstrual cycles, according to results from a study of nearly 3 million women in Sweden.
The study was initiated following widespread anecdotal reports from women who said they experienced changes in their cycles following vaccination, such as fewer days between periods or heavier bleeding.
The researchers did observe an increased risk for postmenopausal bleeding after a third vaccine dose.
The authors noted that “menstrual cycles vary naturally and minor menstrual disturbances are generally not considered to be of clinical importance. Changes can, however, generate considerable distress in the affected women, especially during a mass vaccination campaign when concerns are raised about adverse reactions that might not yet be well characterized.”
The Swedish study is the largest examination to date that looked for a relationship between vaccination and menstrual changes, but researchers found no evidence pointing to the shots as the cause.
The findings were included in this week’s edition of the British Medical Association journal called The BMJ. Data were analyzed for women in Sweden ages 12 to 74 years old from December 2020 to February 2022. More than 2.5 million of the 2.9 million women in the study received at least one dose of a Pfizer, Moderna, or AstraZeneca COVID-19 vaccine. More than half of the women received three doses. Drawing on data from primary care or specialist visits and from hospital stays, the researchers evaluated the impact of the vaccines within 1 to 7 days of getting a shot and within 8 to 90 days. They looked for menstrual disturbance such as a change in cycle days, unusual bleeding, and postmenopausal bleeding.
The researchers said they did see “weak and inconsistent associations” between vaccination and health care contacts related to menstrual changes and postmenopausal bleeding, but there were no patterns that were strong enough to conclude that the shots caused the issues.