Trigger effect

The Covid vaccine can cause short-term changes in period timing and heaviness, but this is probably just the immune system doing its job, researchers say.

- Nicky Pellegrino

When Covid-19 vaccinations were first rolled out, vulnerable elderly people tended to be first in line. Then, as a younger demographic took their turn, there were reports from many women of what seemed like a strange side effect — changes to their menstrual periods. Some noticed cycle length was different after vaccination, others that their period was heavier than usual. By May 2022 in the UK alone, 39,839 women had reported  these sorts of changes.

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.