COVID-19 immunization did not enhance menstrual changes in approximately 3 million Swedish women.
After getting the COVID-19 shot, a lot of women said that their monthly cycle changed, including the number of days they bled and how much blood they lost. Analyses of each vaccine and the risk of bleeding after menopause showed that the risk went up by 23–33% after 8–90 days for Pfizer-BioNTech and after the third dose for Moderna. The link between Oxford and AstraZeneca wasn't as clear. Weak links were found between any amount of immunization and menstrual problems or bleeding in women before menopause (13% or 8% after 1–7 days and 6% or 1% after 8–90 days).
But Swedish researchers at the University of Gothenburg found that when other things were taken into account, almost all of these weak ties disappeared. This means that a cause-and-effect relationship is not possible. "We saw weak and inconsistent links between SARS-CoV-2 vaccination and healthcare contacts for postmenopausal bleeding, and even weaker and less consistent links for menstrual disturbance and premenstrual bleeding," the team wrote in a study published by The BMJ. "These results do not provide strong evidence that SARS-CoV-2 vaccination and healthcare contacts related to menstrual or bleeding disorders are linked," they said. After getting the COVID vaccine between the ages of 12 and 74, 2,946,448 females took part in the study.
Risks for each vaccine type (Pfizer-BioNTech, Moderna, or Oxford-AstraZeneca) and dose (not vaccinated and first, second, and third shots) were looked at during two time periods: 1–7 days (the control period) and 8–90 days. During the time of the study, more than 2.5 million (88%) women got at least one dose of the COVID vaccine. More than 1.6 million (64%) women got three shots. After the third dose, the chance of bleeding went up by 28% in the first seven days and by 25% in the next eight to ninety days.
Controlling for socioeconomic level, history of health care use, and a number of medical problems had little effect on these data. Since these results are based on observations, the experts pointed out that there could be many exceptions. Even though this was a big experiment with nearly full follow-up, the results do not strongly support a link between COVID immunization and diagnoses related to menstruation or bleeding disorders.