Victoria Male, a senior lecturer in reproductive immunology at Imperial College London, first came across the phenomena when a colleague happened to mention her periods being heavier following the vaccine, just as they always were after she had her annual flu jab.
“If anyone should know about the reproductive side effects of vaccines, it ought to be me, but this wasn’t something I’d ever really heard of,” says Male.
She embarked on a small, unfunded study at the same time as other researchers also started looking into it. In Norway, a survey was conducted, while for Male and scientists in the US, menstrual tracking apps were a rich source of data.
“Most people use these apps because they want to know when to expect their next period, so cycle length is the key thing they record,” Male says. “But some get into tracking every little thing.”
The results have been reassuring. The science shows that any changes are typically small, short-term and not a cause for concern.
There is an impact on cycle length. On average, in the month a woman had one dose of the vaccine, her period would be half a day later. A longer delay was seen in those who got two doses during the same cycle – around two to three days.
“But it’s really important to say that even for those who had a longer delay, their periods were normal within two cycles,” says Male.
She also stresses that studies have shown there is no effect on fertility, and a woman is just as likely to get pregnant in the month she is vaccinated against Covid as any other month. Also, there is no increased risk of miscarriage and in fact the vaccine actually seems to reduce the rate of stillbirth.
Researchers did confirm it may affect menstrual flow – 4-7 per cent of women experienced a heavier-than-usual period after vaccination. But the changes are small compared with natural variations in cycles.
“So, slightly later, slightly heavier, and then back to normal within one or two cycles would be the summary of the findings,” says Male.
All the Covid vaccines seem to have this effect, and possibly other vaccines may do, too – the HPV (human papillomavirus) vaccine has been linked to heavy or irregular periods.
Scientists are still trying to piece together exactly why. But rather than it being the result of any particular ingredient, it seems likely it is linked to the activation of the immune system. “We know that innate immune molecules called cytokines can crosstalk with the reproductive system,” says Male, who normally focuses her research on the immune cells in the lining of the uterus that are involved in the build-up and breakdown of tissue.
“One of the things that could be going on here is that when we get vaccinated, we start off what’s called an innate immune response. This is the annoying bit that gives you a sore arm, headache and fatigue, and it sort of triggers the adaptive immune system, which is what provides the protection. We think the innate immune response is probably the bit that is affecting periods.”
Interestingly, women who were on combined oral contraception (oestrogen and progesterone) didn’t report changes to their menstrual periods, whereas those taking the progesterone-only pill did notice a difference, leading researchers to believe that oestrogen is the sex hormone involved in this relationship with the immune system. Also, older women were more likely to have a heavier period, possibly because as we age, our bodies are not as good at tissue repair.
For Male, what all this has highlighted is that menstruation is understudied. “We haven’t done enough research to know how reproductive health interacts with other areas of health, like our immune response.”
