Getting a COVID-19 vaccine isn’t a pleasant experience for everybody. At the very least, you can expect a sore arm, but plenty of people have experienced headaches, fevers, nausea, and a whole host of other symptoms for a day or two after their shots.

One potential side effect has proved a bit controversial so far: changes to a person’s menstrual cycle and periods. It’s been reported in hundreds of thousands of cases following vaccination, but many scientists say there isn’t enough evidence yet to link it definitively to the shot. But according to Dr Victoria Male, a lecturer in immunology and fertility, that can – and should – change.

“Although reported changes to the menstrual cycle after vaccination are short-lived, robust research into this possible adverse reaction remains critical to the overall success of the vaccination program,” wrote Dr Male in an editorial for the BMJ. “Vaccine hesitancy among young women is largely driven by false claims that COVID-19 vaccines could harm their chances of future pregnancy. Failing to thoroughly investigate reports of menstrual changes after vaccination is likely to fuel these fears.”

Overcoming disinformation is a crucial and often difficult part of any vaccination drive, but it’s proving particularly tricky in the fight against the COVID-19 pandemic. That’s partly because the vaccines are all so new, so even though they have been rigorously tested for safety and effectiveness, figuring out the side effects was still to some extent a learning process. Patients who experienced unexpected reactions were invited to report them to programs like the Yellow Card scheme in the UK, or the FDA and CDC in the US. With enough information, researchers hoped, they would be able to sort out the correlation from the causation – the 0.002 percent chance of a blood clot from the rapper’s cousin’s friend’s swollen balls, you might say – and get a good handle on what the vaccines were really responsible for.

Unfortunately, even with these programs in place, making any firm conclusions on menstrual changes can be difficult, explains Dr Male. That’s partly because menstrual cycles can be notoriously unpredictable, but it’s also to do with how data on adverse reactions is collected. Clinical trials don’t include questions on menstrual cycles on their side effects checklist, and participants are “unlikely to report changes to periods unless specifically asked”, she says. That means that different approaches are needed – ones which are “better equipped to compare rates of menstrual variation in vaccinated versus unvaccinated populations,” such as the slew of programs recently commissioned by the National Institutes of Health in the US, says Male.

“If a link between vaccination and menstrual changes is confirmed, this information will allow people to plan for potentially altered cycles,” she writes. “Clear and trusted information is particularly important for those who rely on being able to predict their menstrual cycles to either achieve or avoid pregnancy.”

Crucially on that topic, Male notes that there is no evidence that any COVID-19 vaccine affects fertility – pregnancy and fertility rates are the same for both vaccinated and unvaccinated patients, whether those pregnancies were unintended or clinically assisted. Although she advocates further research into the vaccines’ potential to disrupt periods, she stresses that most menstrual changes reported so far have been temporary, and are unlikely to be connected to the vaccine itself.

“Most people who report a change to their period after vaccination find that it returns to normal the following cycle,” she said. “Menstrual changes have been reported after both mRNA and adenovirus vectored COVID-19 vaccines … [so] if there is a connection, it is likely to be a result of the immune response to vaccination rather than a specific vaccine component.”