The effect of immune stimulation on the menstrual cycle
Research type
Research Study
Full title
The effect of immune stimulation on the menstrual cycle
IRAS ID
353423
Contact name
Victoria Male
Contact email
Sponsor organisation
Imperial College London
Duration of Study in the UK
2 years, 11 months, 31 days
Research summary
For decades, people reported effects of infection or vaccination on their menstrual cycles, but understanding this was never a priority. In 2021, the matter suddenly became urgent, as young women hesitated to protect themselves and others against a pandemic virus because it was unclear whether COVID vaccination altered menstrual cycles and, if so, why this occurred and whether it could have long-term effects on fertility.
In response to these concerns, work undertaken by us and others found that COVID vaccination results in a significantly longer cycle than usual in approximately 1% of individuals, and a significantly heavier period than usual in approximately 4% of individuals. Similar findings have now been made for flu vaccination. These changes are small compared to normal variation and, importantly, reverse within one or two cycles. This is reassuring, but we still don’t know why COVID vaccination affects menstrual cycles in this way.
We already know that immune molecules released after vaccination, called cytokines, affect a gland called the hypothalamus to cause fever. The hypothalamus also determines the length of menstrual cycle, so we will see if vaccination changes how this happens. We will do this by taking blood samples from healthy volunteers who are getting their autumn booster vaccinations and seeing how the cytokines and hormones in their blood compare to people who do not get a vaccination.
We will give participants the option of donating menstrual blood. For participants who do this, we will also look at whether vaccination activates immune cells in the menstruating uterus, which could cause heavier bleeding.
By understanding the mechanism by which immune stimulation affects menstrual cycles, we will be in a better position to counsel people thinking about getting vaccinated, or who experience changes to their menstrual cycles because of infections or other conditions associated with inflammation, such as autoimmune disease.
REC name
HSC REC B
REC reference
25/NI/0151
Date of REC Opinion
28 Oct 2025
REC opinion
Favourable Opinion