Would everyone wearing face masks help us slow the pandemic?

Science ’s COVID-19 reporting is supported by the Pulitzer Center.

As cases of coronavirus disease 2019 (COVID-19) ballooned last month, people in Europe and North America scrambled to get their hands on surgical masks to protect themselves. Health officials jumped in to discourage them, worried about the limited supply of masks for health care personnel. “Seriously people-STOP BUYING MASKS!” began a 29 February tweet from U.S. Surgeon General Jerome Adams. The World Health Organization and U.S. Centers for Disease Control and Prevention (CDC) have both said that only people with COVID-19 symptoms and those caring for them should wear masks.

But some health experts, including the director of the Chinese Center for Disease Control and Prevention, think that’s a mistake. Health authorities in parts of Asia have encouraged all citizens to wear masks in public to prevent the spread of the virus, regardless of whether they have symptoms. And the Czech Republic took the uncommon step last week of making nose and mouth coverings mandatory in public spaces, prompting a grassroots drive to hand make masks.

Even experts who favor masking the masses say their impact on the spread of disease is likely to be modest. Many are also afraid to promote mask buying amid dire shortages at hospitals. But as the pandemic wears on, some public health experts think government messages discouraging mask wearing should shift.

“It’s really a perfectly good public health intervention that’s not used,” argues KK Cheng, a public health expert at the University of Birmingham. “It’s not to protect yourself. It’s to protect people against the droplets coming out of your respiratory tract.”

Cheng and others stress that however masks are used, people must practice social distancing and stay at home as much as possible to prevent the spread of the novel coronavirus. When people do venture out and interact, they’re likely to spew some saliva. “I don’t want to frighten you, but when people speak and breathe and sing—you don’t have to sneeze or cough—these droplets are coming out,” he says.

Although there is some evidence that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can persist in aerosols—fine particles that remain suspended in air—aerosol transmission is likely rare, says Arnold Monto, an epidemiologist at the University of Michigan, Ann Arbor. It’s mostly spread by larger droplets, “and we know that standard surgical face masks will have a modest effect on that kind of transmission,” he says. “When you combine [masks] with other approaches, then they may make a difference.”

Randomized controlled trials focused on other viruses haven’t proved that masking the public decreases infections, though these studies have tended to have small sample sizes, and in many, participants didn’t wear the masks as much as they were instructed to.

Despite messages from some health officials to the contrary, it’s likely that a mask can help protect a healthy wearer from infection, says Benjamin Cowling, an epidemiologist at the University of Hong Kong. Both surgical masks and the more protective N95 respirators have been shown to prevent various respiratory infections in health care workers; there’s been some debate about which of the two is appropriate for different kinds of respiratory infection patient care. “It doesn’t make sense to imagine that … surgical masks are really important for health care workers but then not useful at all for the general public,” Cowling says.

Masks might work better at preventing infection in hospitals than in public, he says, in part because health care workers receive training on how to wear them and because they take other important safety measures such as thorough hand-washing. “I think the average person, if they were taught how to wear a mask properly … would have some protection against infection in the community.”

But the greatest benefit of masking the masses, Cowling and others argue, likely comes not from shielding the mouths of the healthy but from covering the mouths of people already infected. People who feel ill aren’t supposed to go out at all, but initial evidence suggests people without symptoms may also transmit the coronavirus without knowing they’re infected. Data from contact-tracing efforts—in which researchers monitor the health of people who recently interacted with someone confirmed to have an infection—suggest nearly half of SARS-CoV-2 transmissions occur before the infected person shows symptoms. And some seem to contract and clear the virus without ever feeling sick. “If I knew who was asymptomatic and presymptomatic [for COVID-19], I’d … triage the face masks to those individuals,” Monto says. Unfortunately, he adds, “We don’t know who these are.”

A key factor pushing health authorities to discourage mask wearing is the limited supply, says Elaine Shuo Feng, an epidemiologist and statistician at the University of Oxford, whose team last week published in The Lancet a comparison of various health authorities’ face mask recommendations.

For that reason, Mark Loeb, a microbiologist and infectious disease physician at McMaster University, says, “I do not think that it is sound public health policy for people to be going out and purchasing medical masks and N95 respirators and wearing them out on the street.”

The shortage has inspired do-it-yourself movements in many countries to produce cloth masks—which CDC acknowledges can be a last resort for health care workers lacking other protection. Rigorous studies comparing cloth masks to surgical ones or investigating the ideal material for homemade masks are lacking.

Cheng expects masks to become more important in the United States and Europe once the peak of COVID-19 cases passes and social distancing restrictions loosen. “Just imagine you’re traveling in the New York [City] subway on a busy morning. If everyone wears a mask, I’m sure that it would reduce the transmission,” he says, adding, “Don’t ask me to show you a clinical trial that it works.”