You might walk into stores over the next few days and sicken dozens without knowing it. Some might die. Others will think they are dying before they recover.

Do face masks protect against coronavirus? Here's what scientists know so far | David Heymann Read more

That’s the worry I have after reading a paper by Roman Wölfel and colleagues, published this week in Nature. It shows that people are most infectious in the first week after catching Covid-19. During that time they often show no or few symptoms.

In other words, Covid-19 moves like a silent assassin, with unwitting accomplices. Maybe you’ll be one of them. The best way to ensure that you’re not: wear a mask, and keep your distance from others. Don’t wear an N95 respirator, the type in desperately short supply in hospitals, which is designed to keep doctors safe even when doing potentially dangerous medical procedures. But almost any kind of simple cloth covering over your mouth, such as a home-made mask, or even a bandanna, can stop the assassin in its tracks.

The Wölfel paper explains we must focus our efforts on stopping the spread of droplets. This is because the virus is primarily transmitted through tiny droplets of saliva ejected when we speak. You can’t see them, but they are there. We also know that these droplets can go significantly further than the 6ft which is widely cited as a safe distance.

Research supported by Nobel prize-winning virologist Harold Varmus tells us that placing a layer of cloth in front of a person’s face stops 99% of the droplets.

So, the science is clear. We do not know when we are sick. If we are sick, then when we speak we are projecting virus-laden droplets into the air. Wearing a simple cloth mask stops those droplets in their tracks. “I’m not going to wear a surgical mask, because clinicians need those,” said Dr Harvey Fineberg, chair of the National Academy of Sciences’ standing committee on emerging infectious diseases and 21st century health threats. “But I have a nice western-style bandanna I might wear. Or I have a balaclava. I have some pretty nice options.” Fineberg led a committee of experts that has just released an expert consultation explaining that the virus can spread through talking, or even breathing.

Q&A How can I protect myself and others from the coronavirus outbreak? Show Hide The World Health Organization is recommending that people take simple precautions to reduce exposure to and transmission of the coronavirus, for which there is no specific cure or vaccine. The UN agency advises people to: Frequently wash their hands with an alcohol-based hand rub or warm water and soap

Cover their mouth and nose with a flexed elbow or tissue when sneezing or coughing

Avoid close contact with anyone who has a fever or cough

Seek early medical help if they have a fever, cough and difficulty breathing, and share their travel history with healthcare providers

Advice about face masks varies. Wearing them while out and about may offer some protection against both spreading and catching the virus via coughs and sneezes, but it is not a cast-iron guarantee of protection Many countries are now enforcing or recommending curfews or lockdowns. Check with your local authorities for up-to-date information about the situation in your area. In the UK, NHS advice is that anyone with symptoms should stay at home for at least 7 days. If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home.

Prof David Heymann CBE, a World Health Organization (WHO) adviser, said, “I think that wearing a mask is equally effective or more effective than distancing.”

In a paper published in Nature on Friday, a five-year study from the University of Hong Kong and the University of Maryland has found that a simple non-fitted mask blocked 100% of coronavirus droplets and aerosol. There’s a vast chasm between what the science is showing and what many countries are doing. Masks may be the most important weapon in our war on the virus. But we’re not even using it.

Worse, key leaders continue to lessen its importance. The WHO recommends only wearing a mask if you are sick, or looking after somebody who is sick. With the new evidence, this makes no sense, if it ever did. Many people who are infected don’t even know it. No wonder so many people are expressing such confusion about whether they should wear a mask.

We can see dramatic results when a country changes its policy on masks

Other key data offers even more proof. Every country with enforced mask usage shows dramatically lower death rates compared with countries not using masks widely.

We can also see dramatic results when a country changes its policy on masks. Masks were hard to come by in South Korea until late February. Then the government stepped in and ensured a supply for every person in the country. Up until then, South Korea showed a similar-shaped exponential trajectory to Italy. After that point the exponential growth slowed, and today the number of active cases is decreasing. There is no economic lockdown there.

Stanford economic policy research experts state that “the firm recommendation against masks in community settings appears incompatible with the available evidence”. Modeling by Yale researchers estimates that “the benefits of each additional cloth mask worn by the public are conservatively in the $3,000-$6,000 range due to their impact in slowing the spread of the virus”. A cloth mask is basically free, since you can make one from old T-shirts or sheets. So the economic payoff is hard to question. What other investment pays off by over 1,000 to 1?

One huge challenge is that mask wearing only really pays off when most people do it. A Food and Drug Administration analysis of the flu estimates that if 50% of the population uses a mask, virus transmission is reduced by half. If 80% of the population uses a mask, the virus is “essentially eliminated”.

That is why many jurisdictions have adopted laws that require mask use whenever in crowded places, such as public transport or shops. Some jurisdictions go further and require masks whenever out in public.

“Masks for All laws” are now in place in Israel, Austria, the Czech Republic, Hong Kong, Mongolia and elsewhere, with more locations added every day. In the United States, where these laws are taking a long time to appear, some counties are going it alone, including Laredo in Texas and Riverside county in California, which have created their own local mask use laws. New York and Texas haven’t yet used legislation, but their leadership have made a clear and direct plea to their communities to always wear a mask when in public.

These steps don’t replace the need for social distancing and hand-washing. We need to use the entire set of tools we have in our toolbox to stop this killer.

If we can’t rely on our governments to take this step, then we will have to take things into our own hands. We must rely on grassroots community efforts to get up to that magic 80% compliance number. We know this is possible, because the Czech Republic did it last month. In that country, a brilliantly effective social media and influencer campaign saw the country go from 0% mask use to nearly 100% mask use in three days. Then the government followed and made it the law.

Now it’s time for the rest of us to make this happen in our own communities.