Recent studies have revealed that COVID-19 vaccines may have a temporary but noticeable effect on women's menstrual cycles and their accompanying symptoms. A study of nearly 4,000 women in the US found that their menstrual cycle lengths were extended by 0.7 day after the first dose and 0.9 day after the second dose. Similarly, a larger study of nearly 20,000 women in the UK found that women who got both doses of a vaccine within the same menstrual cycle had their cycle length extended by an average of 3.7 days. Additionally, a paper published in the Journal of Infection and Chemotherapy found that the average difference in cycle length before and after vaccination could be as high as 2.5 days.
In addition to a change in cycle length, women may experience a range of symptoms accompanying their periods. A study of nearly 5,000 women in six Arab countries found that vaccinated people reported back pain, nausea, tiredness, pelvic pain, passage of loose stools, heavier flow, and more days of bleeding.
At this time, the effects of the vaccine on fertility and reproductive health are still being investigated. However, the potential benefits of vaccination may outweigh the risks, as pregnant people who are not vaccinated may be at higher risk for hospitalization, critical care admissions, and morbidity rates than their vaccinated counterparts. As for COVID-19 infection, some studies suggest that it may not affect cycle length, though a small number of people may experience cycle changes after infection.