COVID-19 vaccination may be associated with short-term lengthening of the menstrual cycle for some women, according to a new study.

The findings are drawn from 3,858 female nurses in the United States and Canada who have been filling out questionnaires about their periods twice a year since 2011. As of December 2021, 91 per cent of them had been vaccinated against the coronavirus. Before the pandemic, 15 per cent reported irregular cycles; that rose to 22.7 per cent in 2021, the researchers reported on Wednesday (July 13) in the American Journal of Obstetrics & Gynecology.

NORMAL MENSTRUAL CYCLE

Vaccinated women had a 54 per cent higher risk of increased cycle length compared to unvaccinated women, regardless of vaccine type and even after taking pandemic stress and health-related factors into account, the report said. On closer analysis, vaccination was only associated with change to longer cycles in the first six months after vaccination and among women whose cycles were short, long or irregular before vaccination, not among women with normal length, regular cycles.

“A normal menstrual cycle is characterised by tightly regulated inflammatory and immune mediators” that may be temporarily affected by the body’s immune response to the vaccines, the researchers said. They call for monitoring of “menstrual cycle health in vaccine clinical trials and increased attention to sex-based differences in vaccine response”.