In Japan, examination of menstrual abnormalities after vaccination with mRNA-based COVID-19 vaccine using menstrual cycle tracking app

In Japan, since April 2021, the vaccination with the mRNA-based COVID-19 vaccine [BNT162b2 (Pfizer/BioNTech) and mRNA-1273 (Moderna)] has started, an increasing number of women are coming to see gynecologists with chief complaints of irregular bleeding, menstrual pain, and irregular menstruation.

Menstrual abnormalities (irregular menstruation, irregular bleeding, etc.) have already been reported in other countries as one of the side effects of the mRNA-based COVID-19 vaccine. However, in Japan, there are no research reports investigating the relationship between mRNA-based COVID-19 vaccination and menstrual abnormalities to date. Therefore, it is not clear how often menstrual abnormalities occur in all women who have been vaccinated with mRNA-based COVID-19 vaccine, how long they occur, and which women are more likely to have menstrual abnormalities. Therefore, in our clinical research, we used a menstrual cycle tracking app developed by MTI Co., Ltd., which has knowledge of big data utilization related to women's healthcare, to investigate the details of menstrual abnormalities after mRNA-based COVID-19 vaccination. A menstrual cycle tracking app can collect real-time menstrual data and also allow subjects in this clinical study to accurately report changes in their cycles.

A clinical study using data from the app revealed that women who received one dose of the mRNA-based COVID-19 vaccine during their menstrual cycle delayed their next period by about half a day. Furthermore, women who received two doses of the mRNA-based COVID-19 vaccine within the same period were found to delay their next menstrual period by about three days. Importantly, women who received a second dose of the mRNA-based COVID-19 vaccine within one menstrual cycle were found to return to normal menstrual timing within two menstrual cycles.

Furthermore, about 4% of all subjects had worse-than-usual menstrual periods after receiving the mRNA-based COVID-19 vaccine but returned to normal periods in the next menstrual cycle. In other words, it was revealed that vaccination with the mRNA-based COVID-19 vaccine temporarily disrupted the menstrual cycle.

To date, the mechanism by which vaccination with mRNA-based COVID-19 vaccine causes abnormal menstrual cycles has not been elucidated. It is hypothesized that vaccination with the mRNA-based COVID-19 vaccine affects immune cells that help repair endometrial tissue. Older people are more likely to have heavier menstrual cycles than usual because vaccination with the mRNA-based COVID-19 vaccine slows the repair of endometrial tissue. Further clinical studies should be conducted to dissect physiological abnormalities following vaccination with mRNA-based COVID-19 vaccine.

I do not have potential conflicts of interest.

Published in JAMA on May 26, 2023 by Kyoto@takumaH

Doctor specializing in cancer care.
Doctor specializing in emerging infectious diseases.