Vaccination against Covid-19 can temporarily change the length of people's menstrual cycles, shows a new study of nearly 20,000 women published in the British Medical Journal (BMJ), confirming results from previous results.
Shortly after the launch of the Covid-19 vaccines, vaccinated women from all over the world took to social media to post about changes in their menstrual cycle – a phenomenon dubbed "period weirdness" for lack of an official diagnosis.
Now, research published in the British Medical Journal shows that vaccinated women had their periods delayed by an average of about one day, compared to those who were not vaccinated. While a cycle generally lasts about 28 days, it varies from person to person, but it can also change during periods of stress or changes in lifestyle.
The data from the study, published on Tuesday 27 September 2022, comes from a menstrual tracking application called Natural Cycles – an app validated by the US Food and Drug Administration (FDA) and often used by women who wish to monitor their fertility.
The data showed that vaccinated people had their periods on average 0.71 days later after the first dose of the vaccine. Those who received two doses of vaccine in the same menstrual cycle experienced greater disturbances. In this group, the average increase in cycle length was four days. 13% even experienced a delay of eight days or more, compared to 5% in the group of women who were not vaccinated.
"For most women, the effects were only temporary and lasted for one cycle before returning to normal," said Alison Edelman, Professor of obstetrics and gynaecology at Oregon Health & Science University who led the study. She added that there was no evidence that the side effects of menstruation had any impact on fertility.