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Wearing Masks to Prevent Coronavirus: Here’s What Scientists Know

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With the coronavirus on the march through much of the United States, scientists are urging Americans to adopt the few health measures shown to slow the virus: universal mask use, social distancing, good ventilation indoors and hand hygiene.

Mask-wearing has become a particularly divisive — and partisan — issue over the past few months. Still, faced with a surge in cases, 40 states, including recent holdouts like Iowa and North Dakota, now have put mask requirements in place.

Among public health experts, there is near-unanimous endorsement of universal mask mandates to shield people from the virus and slow the pandemic.

“The more people who wear a mask, the more the community is protected and therefore the more you individually benefit,” said Dr. John Brooks, chief medical officer of the Centers for Disease Control and Prevention’s Covid-19 response program. “It’s like a herd effect.”

Dr. Brooks is the architect of a recent agency bulletin saying that masks benefit wearers, not just those around them. Increasing the proportion of people who wear masks by 15 percent could prevent the need for lockdowns and cut economic losses that may reach $1 trillion, about 5 percent of gross domestic product, the C.D.C. said.

“If it can’t help us avert the shutdowns that are happening, it can certainly help us come down the backside of this peak and then keep things down,” Dr. Brooks said.

Other experts were careful to note that masks cannot work in isolation. “Controlling community spread of Covid-19 and protecting individuals requires a multitiered approach,” said John Volckens, a public health engineer at Colorado State University in Fort Collins.

In August, Dr. Volckens organized a workshop for the National Academies of Sciences, Engineering and Medicine on ways to prevent airborne transmission of the virus. “Masks are a critical part of that approach,” he said. “That is definitely the consensus among scientists.”

So what is the evidence supporting mask use? And what about that Danish study questioning whether masks protect the wearer? We asked experts to weigh in on the latest evidence.

The term mask refers to any kind of protective facial covering, but its effectiveness depends greatly on the type. The gold standard is the N95 respirator which, as its name suggests, can block 95 percent of harmful particles a wearer might breathe in or out. Surgical masks are also highly effective at filtering these particles.

But experts say only health care workers require gold-standard protection. Doctors and nurses work closely with infected patients for prolonged periods, which significantly increases their risk of infection with the coronavirus, Dr. Brooks noted.

The average person, on the other hand, is exposed to much less virus and less often, and so can be protected with a well-made cloth covering, Dr. Brooks said. The best cloth face coverings, which have multiple layers that can trap viral particles — the thickest are mostly impervious to light — are as effective as surgical masks in some circumstances.

Cloth masks are also reusable and durable, and even after regular washings, they maintain their effectiveness. N95s and surgical masks are usually worn once and “end up in a landfill,” Dr. Brooks said.

It’s indisputable that N95 respirators and surgical masks prevent pathogens from infecting others — one reason doctors have traditionally worn surgical masks to protect their patients.

There is increasing evidence that cloth face coverings, too, stop virus expelled by an infected person when breathing, talking, singing or shouting — controlling the spread at the source.

This discovery became especially important once scientists learned that people who don’t even feel symptoms may spread the virus. More than 50 percent of all infections may be transmitted by asymptomatic people.

Apart from epidemiological studies showing that mask use is high in countries that have successfully controlled the virus, mask mandates have been shown to significantly slow the virus in U.S. states and in health care settings, Dr. Volckens said.

All kinds of masks offer the wearer some degree of protection, multiple studies have shown. Exactly how much protection is not yet clear.

“The protection for the wearer is not 100 percent,” Dr. Leana Wen, the former assistant health commissioner of Baltimore, said of cloth masks. “That’s also why universal masking is important, because we need the people who are infected to be wearing it.”

N95 masks are thought to be the most effective in this regard, followed by surgical masks. But evidence for benefit from cloth masks is scarce.

“There haven’t been good studies on protecting the wearer,” said Linsey Marr, an expert at Virginia Tech on the airborne transmission of viruses. Still, she added, most researchers assume cloth masks provide at least some protection.

Some of the studies on wearer protection were conducted in labs under ideal conditions, or with mannequins, which does not capture more realistic situations in which people wear masks inconsistently or incorrectly.

Still, in one such study, a mask made of four layers of tightly woven surgical gauze prevented transmission more effectively when worn by both parties than when worn only by an infected person, Dr. Brooks said.

“They worked best together, and the cloth masks performed essentially as well as surgical masks,” he said. “When you’re wearing a mask, you’re protecting others as well as yourself.”

Critics of mask-wearing measures have long demanded a randomized clinical trial that establishes their effectiveness. But while such trials are the standard for drugs and vaccines, they are not ideal for evaluating behaviors subject to people’s recall, experts said.

“Show me the clinical trials that showed the efficacy of hand washing,” Dr. Volckens said. “And I think we all agree that smoking causes cancer and is bad for you — does that mean that we can’t believe that smoking causes cancer because there isn’t a clinical trial?”

Most studies on cloth face coverings have been observational and looked at whether their use stopped the spread at a community level. The C.D.C.’s latest bulletin on masks lists several such studies confirming the benefit of universal masking mandates.

In one study, two masked hair stylists had symptoms but did not transmit the virus to any of their 67 masked clients. In another, face coverings seemed to reduce risk of infection by 70 percent during an outbreak aboard the U.S.S. Theodore Roosevelt.

It’s particularly difficult to assess a mask’s benefit to the wearer because “you’ve got to be able to measure what’s behind the mask and what’s getting through the mask into their mouth,” Dr. Marr said.

A Danish study published on Wednesday was a randomized clinical trial assessing whether a mask protected wearers. It found no statistically significant effect. But the study has serious limitations, experts said: It was conducted when community transmission in Denmark was low, and masks were far from the norm.

“It didn’t get at this communitywide effect,” Dr. Volckens said.

The numbers in the study were small, and only half of the mask-wearing subjects reported doing so as strictly as recommended. Even those who wore them regularly would not have worn them at restaurants, bars, gyms or in their homes — settings responsible for the majority of spread in a community, Dr. Marr noted.

“It’s hard to do these studies in real life,” she said.

The study still found a 15 percent protection for the wearer, although the figure was not statistically significant. But it may be an underestimate, Dr. Marr and other researchers said.

“I still think masks are the most cheap, effective, versatile intervention that we can have if social distancing is not possible or variable, or if indoor ventilation is poor,” said Julian Tang, honorary professor of respiratory sciences at the University of Leicester in the United Kingdom.

The C.D.C. has been criticized for an about-face on masks since the beginning of the pandemic, when it urged only symptomatic people to wear them. The agency did not recommend universal use of face coverings until April. (The World Health Organization was even later, issuing its endorsement in July.)

The agency was reluctant to recommend masks at first because it worried about a run on the high-quality masks needed for health care workers, Dr. Marr said. “I think it took time to realize that there’s different objectives in the health care setting versus in the community,” she said.

It is natural when dealing with an unknown virus for recommendations to change significantly over time, Dr. Wen said. “We know a lot more now, and I think we should acknowledge that we were wrong,” she said.

Still, changes in guidance should be seen as a sign that the policy is following the latest science, she added.

“Somehow that change has been framed by some people as public health experts not knowing what we’re doing,” Dr. Wen said. “But actually, we should see this change in guidance as part of the necessary evolution. That’s the cornerstone of a solid public health response.”

For example, it was not clear at first that the virus could be transmitted by air, especially indoors and by people without symptoms. So scientists assumed that the new coronavirus behaved like the coronaviruses causing SARS and MERS.

But the C.D.C. was quick to recommend masks once it was clear that asymptomatic transmission was a big contributor, Dr. Brooks said: “Science changes. So do we, and so do our recommendations.”

Likewise, the agency initially recommended masks only to protect those near an infected person because “that’s where we had the earliest and clear data.” Now there is enough evidence to say that masks also benefit the wearer.

“Our guidance has not changed — we are recommending everybody wear masks,” he said. “What has changed is we can now give you a reason. a personal reason that will motivate people.”

The experts all emphasized that mask use is just one tool that can slow the pandemic. Social distancing, ventilation and hand hygiene are also important.

“None of those is 100 percent effective by itself,” Dr. Marr said. “But when we combine them, then we can make a big dent in the risk of transmission.”

Masking is also among the easiest of community strategies to adopt, or should be, as states all over the country try to avert lockdowns, Dr. Brooks said: “We believe strongly that universal masking policies can help avert shutdowns.”

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