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

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.”

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.

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.

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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