Heavy Bleedings and Other Menstrual Disturbances in 18- to 30-Year-Old Women after COVID-19 Vaccination

Background: Many signals of menstrual disturbances as possible side effects of vaccination against COVID-19 have been reported. Our objective was to estimate the association between vaccination and the occurrence of such disturbances among women aged 18-30 years in Norway.Methods: We used electronic questionnaires to collect reports of menstrual disturbances from 3972 women aged 18-30 years, participating in the population-based Norwegian Young Adult Cohort. We estimated the relative risk of menstrual disturbances according to vaccination using a self-controlled case-series design. We examined the occurrence of disturbances before and after both the first and second doses of COVID-19 vaccine, vaccine type/brand, previous gynaecological conditions and use of contraception/ hormone treatment.Findings: The relative risk of reporting a heavier bleeding than usual during the first menstrual cycle after the first vaccine dose was 1.90 (95% CI: 1.69-2.13), while it was 1.84 (1.66-2.03) after the second dose. Increased risks of other menstrual disturbances after vaccination were also observed. We found no significant differences in the relative risk of menstrual disturbances according to vaccine type/brand, hormonal contraception status nor to the presence of gynecological condition(s).Interpretation: Menstrual disturbances were generally common regardless of vaccination. We found a significant increase in menstrual disturbances after vaccination, particularly for heavier bleeding than usual, longer duration and for short interval between menstruations. In the future, menstrual characteristics should be routinely included in vaccine trials. Potential mechanisms for the observed disturbances should be explored.Funding: This work was supported by the Norwegian Ministry of Health through a programme for corona vaccination surveillance.Declaration of Interest: We declare no competing interests.Ethical Approval: The study was approved by the Regional Ethics Committee South East Norway, no. 229359, 19 February 2021.