We all want to protect ourselves from coronavirus – but we need to make sure the things we’re doing are effective. There are several measures we can all take to reduce the spread of Covid-19, including physical distancing, thorough handwashing, keeping surfaces clean, protecting the most vulnerable by staying home, and isolating ourselves if we have symptoms. We know from scientific evidence, as well as what we have learned from other countries further ahead in their epidemics, that these things work.

But what does the evidence say about how well face masks work, and who should wear them?

When someone with Covid-19 wears a face mask, they can prevent other people from being infected. It’s also vital for healthcare workers to wear face masks (and to have masks available to wear), both during surgery and routine procedures, to prevent patients becoming infected. Health workers are highly trained experts who know how to correctly use face masks. The masks they use are medical-grade equipment, and are far more effective than their cloth alternatives.

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But what about the public?

We can’t automatically assume that because face masks work in hospitals, they will work everywhere. The reason masks are effective in hospitals is partly because they are changed often and correctly fitted, and partly because health workers know how to remove the mask without becoming infected from their outer surface, which could harbour viruses.

There is a lack of good, robust evidence on the effectiveness of standard face masks worn by the public. One of the best forms of evidence in medical research is a randomised controlled trial. Typically, these are “blinded”, where information that may influence participants is withheld until after the experiment is completed. We would have difficulty doing this kind of trial of face masks, because it’s impossible for participants to be unaware of whether or not they’re wearing a mask.

We don’t even have good, case-controlled studies about how effective face masks are at preventing the spread of influenza, which is the model for respiratory virus diseases. There have been some studies comparing a group of people who got flu with a group of people who didn’t get flu , which asked retrospectively whether they wore face masks, but they don’t convincingly tell us that it was the face mask, rather than something else, that was effective in preventing transmission of flu.

But of course that’s no reason not to ask the question. We’ve recently seen scientists putting forward hypotheses based on lab experiments which simulate coughs and sneezes and measure droplet spread. These studies suggest it may be possible for droplets to travel farther than was previously thought, which may indicate that the distance recommended for physical distancing should be reassessed. Crucially, this was not a real-life experiment, so we can’t be sure how much this artificial setting represents an accurate picture of reality.

Scientists are also working to gather more evidence about whether people who have the virus but don’t yet have symptoms can actually transmit the disease to other people before they develop them. At the moment we’re not sure about this – but when we do know, this research could indicate whether people who are infected but asymptomatic should wear masks. New, rigorous studies are desperately needed on this, but as yet we just don’t have strong evidence.

It’s reasonable to ask whether, even in the absence of strong evidence, people should just wear masks anyway as a precaution. We all cough, and common sense suggests that it’s harder to cough on somebody else if you have a mask over your nose and mouth.

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But it’s not always wise to do something in the absence of evidence – especially if removing a mask that has been contaminated may increase your risk of infection. As medical staff and patients in hospital are the people who benefit most from face masks, encouraging the mass adoption of masks could reduce the supply for the people who need them most.

Some scientists also suggest that wearing a mask may lead to a false sense of security. The last thing we want is people neglecting the measures we know definitely work, such as washing hands, keeping surfaces clean and maintaining distance from other people, because they thought a face mask would keep them safe.

These are some of the many factors that the World Health Organization will take into account when it considers the recommendations about face masks put to it by independent advisory committees, including the one that I chair. Evidence will be crucial in what it decides about whether the public should be wearing face masks to protect against coronavirus. In the meantime, there are plenty of things we can do now to keep ourselves safe and healthy. But for now, the jury on face masks is still out.

• David Heymann is professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine