Since the first COVID-19 vaccines were made widely available in early 2021, some women have anecdotally noted changes to their menstrual cycle following vaccination, reporting shorter intervals between menses and heavier bleeding.
Some studies that relied on menstrual tracking apps and self-reports have also noted a relationship between vaccination and menstrual disturbances.
Today in the BMJ researchers in Sweden used electronic health registry data to evaluate the risks of menstrual disturbance and bleeding after COVID-19 vaccination in 2,946,448 women aged 12 to 74 years from December 2020 to February 2022. This is the largest study to address the relationship between COVID-19 vaccines and menstrual cycles or postmenopausal bleeding.
The primary outcomes measured by this study were primary care visits, specialist outpatient visits, and days of hospital stay related to menstrual disturbance or bleeding before or after menopause. During the study, 2,580,007 women (87.6%) received at least one SARS-CoV-2 vaccination, and among the vaccinated, 1,652,472 (64.0%) of 2,580,007 women received three doses.
For women with menstrual disturbances, researchers noted which vaccine was used (Pfizer/BioNTech, Moderna, or Oxford-AstraZeneca), which dose (unvaccinated and first, second, and third dose) was connected to a menstrual disturbance, and over which of two time windows (1 to 7 days, considered the control period, and 8 to 90 days) the menstrual disturbance took place.
The authors explained that the 90 days was for women to note an excessive bleeding disturbance and obtain an appointment with a gynecologist.
Associations only weak
A risk of postmenopausal bleeding was seen in the study, with a 23% to 33% increased risk after 8 to 90 days with the Pfizer and Moderna vaccines after a third dose.
For postmenopausal women, the risk at 1 to 7 days was 28% for bleeding and at 25% in the 8- to 90-day risk window. Overall, the adjusted hazard ratio for premenopausal bleeding after vaccination with any dose compared with unvaccinated periods was 1.08 (95% confidence interval, 0.90 to 1.30) in the 1- to 7-day risk windows and 1.01 (95% CI, 0.91 to 1.12) for the 8- to -90-day risk window, the authors said.
For menstrual disturbance, adjustment for covariates almost completely removed the associations found after vaccination.
The risk of abnormal bleeding in premenopausal women was much lower. Risk of bleeding after any dose of vaccine was 13% or 8% after 1 to 7 days, and 6% and 1% after 8 to90 days, respectively, for the Pfizer and Moderna vaccines.
"For menstrual disturbance, adjustment for covariates almost completely removed the associations found after vaccination in the crude estimates, and only a weak association remained after the first dose, limited to the one to seven days risk window, " the authors concluded. "Considering the characteristics of this condition, and that the change is measured based on encounters with specialist healthcare in this study, a causal effect limited to this risk window is unlikely."
