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When her Danish colleagues first suggested distributing protective fabric face masks to people in Guinea-bissau to stem the spread of the coronavirus, Christine Benn wasn't so sure.

"I said, 'Yeah, that might be good, but there'south limited information on whether face masks are actually constructive,'" says Benn, a global-health researcher at the University of Southern Kingdom of denmark in Copenhagen, who for decades has co-led public-health campaigns in the West African country, one of the world's poorest.

That was in March. But by July, Benn and her team had worked out how to possibly provide some needed data on masks, and hopefully help people in Republic of guinea-Bissau. They distributed thousands of locally produced cloth face coverings to people as part of a randomized controlled trial that might be the globe's largest test of masks' effectiveness confronting the spread of COVID-19.

Face masks are the ubiquitous symbol of a pandemic that has sickened 35 million people and killed more than 1 million. In hospitals and other health-intendance facilities, the use of medical-grade masks conspicuously cuts down transmission of the SARS-CoV-2 virus. But for the diverseness of masks in use past the public, the data are messy, disparate and often hastily assembled. Add to that a divisive political discourse that included a US president disparaging their use, just days before existence diagnosed with COVID-19 himself. "People looking at the evidence are understanding it differently," says Baruch Fischhoff, a psychologist at Carnegie Mellon University in Pittsburgh, Pennsylvania, who specializes in public policy. "It's legitimately disruptive."

To be clear, the science supports using masks, with recent studies suggesting that they could save lives in unlike ways: research shows that they cut downwards the chances of both transmitting and catching the coronavirus, and some studies hint that masks might reduce the severity of infection if people do contract the disease.

Simply being more than definitive most how well they work or when to use them gets complicated. There are many types of mask, worn in a variety of environments. There are questions near people's willingness to vesture them, or wear them properly. Even the question of what kinds of study would provide definitive proof that they work is hard to answer.

"How good does the evidence need to be?" asks Fischhoff. "It'due south a vital question."

Beyond golden standards

At the start of the pandemic, medical experts lacked good evidence on how SARS-CoV-2 spreads, and they didn't know plenty to make strong public-health recommendations well-nigh masks.

The standard mask for use in health-care settings is the N95 respirator, which is designed to protect the wearer by filtering out 95% of airborne particles that measure 0.3 micrometres (µm) and larger. As the pandemic ramped up, these respirators apace fell into short supply. That raised the now contentious question: should members of the public carp wearing basic surgical masks or textile masks? If so, under what conditions? "Those are the things nosotros usually [sort out] in clinical trials," says Kate Grabowski, an infectious-disease epidemiologist at Johns Hopkins School of Medicine in Baltimore, Maryland. "But we just didn't have time for that."

So, scientists take relied on observational and laboratory studies. There is also indirect evidence from other infectious diseases. "If you look at any 1 paper — information technology'south not a slam dunk. Merely, taken all together, I'one thousand convinced that they are working," says Grabowski.

Confidence in masks grew in June with news about two hair stylists in Missouri who tested positive for COVID-nineteen¹. Both wore a double-layered cotton face up covering or surgical mask while working. And although they passed on the infection to members of their households, their clients seem to have been spared (more than than one-half reportedly declined gratuitous tests). Other hints of effectiveness emerged from mass gatherings. At Black Lives Matter protests in United states of america cities, nearly attendees wore masks. The events did not seem to trigger spikes in infections², all the same the virus ran rampant in late June at a Georgia summertime camp, where children who attended were not required to wearable face coverings³. Caveats abound: the protests were outdoors, which poses a lower chance of COVID-19 spread, whereas the campers shared cabins at night, for example. And because many non-protesters stayed in their homes during the gatherings, that might take reduced virus transmission in the customs. Nevertheless, the anecdotal testify "builds up the picture", says Theo Vos, a wellness-policy researcher at the University of Washington in Seattle.

More-rigorous analyses added direct evidence. A preprint written report^four posted in early August (and not nevertheless peer reviewed), establish that weekly increases in per-capita mortality were four times lower in places where masks were the norm or recommended by the government, compared with other regions. Researchers looked at 200 countries, including Mongolia, which adopted mask utilize in January and, every bit of May, had recorded no deaths related to COVID-19. Another report⁵ looked at the effects of US state-government mandates for mask use in April and May. Researchers estimated that those reduced the growth of COVID-19 cases by up to 2 percentage points per day. They charily advise that mandates might have averted as many as 450,000 cases, afterward controlling for other mitigation measures, such as physical distancing.

"You don't have to do much math to say this is obviously a practiced idea," says Jeremy Howard, a research scientist at the University of San Francisco in California, who is function of a team that reviewed the evidence for wearing face masks in a preprint article that has been widely circulated⁶.

But such studies practise rely on assumptions that mask mandates are being enforced and that people are wearing them correctly. Furthermore, mask utilize often coincides with other changes, such as limits on gatherings. As restrictions lift, farther observational studies might begin to split the impact of masks from those of other interventions, suggests Grabowski. "It will become easier to see what is doing what," she says.

Although scientists can't command many confounding variables in human populations, they can in animal studies. Researchers led by microbiologist Kwok-Yung Yuen at the University of Hong Kong housed infected and salubrious hamsters in adjoining cages, with surgical-mask partitions separating some of the animals. Without a barrier, nigh two-thirds of the uninfected animals caught SARS-CoV-ii, according to the newspaper^seven published in May. But simply nigh 25% of the animals protected by mask material got infected, and those that did were less sick than their mask-free neighbours (as measured by clinical scores and tissue changes).

The findings provide justification for the emerging consensus that mask employ protects the wearer likewise every bit other people. The piece of work also points to some other potentially game-changing thought: "Masking may not only protect you from infection only besides from severe illness," says Monica Gandhi, an infectious-illness physician at the University of California, San Francisco.

Gandhi co-authored a paper⁸ published in late July suggesting that masking reduces the dose of virus a wearer might receive, resulting in infections that are milder or even asymptomatic. A larger viral dose results in a more aggressive inflammatory response, she suggests.

She and her colleagues are currently analysing hospitalization rates for COVID-19 before and after mask mandates in 1,000 The states counties, to determine whether the severity of disease decreased after public masking guidelines were brought in.

The idea that exposure to more virus results in a worse infection makes "absolute sense", says Paul Digard, a virologist at the Academy of Edinburgh, UK, who was non involved in the research. "Information technology'southward some other argument for masks."

Gandhi suggests another possible benefit: if more people get balmy cases, that might aid to enhance immunity at the population level without increasing the brunt of severe illness and death. "As we're awaiting a vaccine, could driving up rates of asymptomatic infection practise expert for population-level immunity?" she asks.

Back to ballistics

The masks argue is closely linked to another divisive question: how does the virus travel through the air and spread infection?

The moment a person breathes or talks, sneezes or coughs, a fine spray of liquid particles takes flight. Some are large — visible, even — and referred to as droplets; others are microscopic, and categorized as aerosols. Viruses including SARS-CoV-2 hitch rides on these particles; their size dictates their behaviour.

Aerosol can shoot through the air and land on a nearby person's optics, nose or oral fissure to cause infection. But gravity speedily pulls them downward. Aerosols, by contrast, can bladder in the air for minutes to hours, spreading through an unventilated room similar cigarette smoke.

Time-lapse images testify how cough aerosol spread from a person wearing an N95 mask that has a valve to expel exhaled air. Credit: S. Verma et al./Phys. Fluids

What does this imply for the ability of masks to impede COVID-19 transmission? The virus itself is only most 0.1 µm in diameter. Merely considering viruses don't leave the body on their own, a mask doesn't need to block particles that modest to exist effective. More relevant are the pathogen-transporting droplets and aerosols, which range from virtually 0.ii µm to hundreds of micrometres beyond. (An average human pilus has a diameter of about 80 µm.) The bulk are one–10 µm in diameter and tin linger in the air a long fourth dimension, says Jose-Luis Jimenez, an environmental chemist at the University of Colorado Boulder. "That is where the action is."

Scientists are still unsure which size of particle is most of import in COVID-19 manual. Some can't even hold on the cut-off that should define aerosols. For the aforementioned reasons, scientists still don't know the major form of transmission for flu, which has been studied for much longer.

Many believe that asymptomatic transmission is driving much of the COVID-nineteen pandemic, which would suggest that viruses aren't typically riding out on coughs or sneezes. By this reasoning, aerosols could prove to be the almost important transmission vehicle. So, it is worth looking at which masks can stop aerosols.

All in the fabric

Even well-plumbing equipment N95 respirators autumn slightly curt of their 95% rating in existent-world use, actually filtering out around 90% of incoming aerosols downwards to 0.three µm. And, co-ordinate to unpublished research, N95 masks that don't have exhalation valves — which expel unfiltered exhaled air — block a similar proportion of outgoing aerosols. Much less is known about surgical and fabric masks, says Kevin Fennelly, a pulmonologist at the United states of america National Heart, Lung, and Blood Institute in Bethesda, Maryland.

In a review⁹ of observational studies, an international research team estimates that surgical and comparable cloth masks are 67% effective in protecting the wearer.

In unpublished work, Linsey Marr, an environmental engineer at Virginia Tech in Blacksburg, and her colleagues found that fifty-fifty a cotton wool T-shirt can cake half of inhaled aerosols and well-nigh eighty% of exhaled aerosols measuring 2 µm across. Once y'all get to aerosols of 4–5 µm, almost any fabric tin can block more than fourscore% in both directions, she says.

Multiple layers of fabric, she adds, are more constructive, and the tighter the weave, the ameliorate. Another study¹⁰ found that masks with layers of different materials — such as cotton and silk — could catch aerosols more efficiently than those made from a single material.

Benn worked with Danish engineers at her university to test their two-layered fabric mask blueprint using the aforementioned criteria as for medical-class ventilators. They found that their mask blocked only eleven–xix% of aerosols downward to the 0.3 µm mark, according to Benn. Simply because almost manual is probably occurring through particles of at to the lowest degree 1 µm, co-ordinate to Marr and Jimenez, the actual difference in effectiveness betwixt N95 and other masks might not be huge.

Eric Westman, a clinical researcher at Duke Academy School of Medicine in Durham, Due north Carolina, co-authored an August written report¹¹ that demonstrated a method for testing mask effectiveness. His team used lasers and smartphone cameras to compare how well fourteen different textile and surgical face coverings stopped droplets while a person spoke. "I was reassured that a lot of the masks we use did work," he says, referring to the performance of cloth and surgical masks. Just thin polyester-and-spandex neck gaiters — stretchable scarves that tin can be pulled up over the mouth and nose — seemed to actually reduce the size of droplets beingness released. "That could be worse than wearing nothing at all," Westman says.

Some scientists propose non making too much of the finding, which was based on just 1 person talking. Marr and her team were among the scientists who responded with experiments of their ain, finding that cervix gaiters blocked virtually big droplets. Marr says she is writing up her results for publication.

"There's a lot of information out at that place, merely it's disruptive to put all the lines of evidence together," says Angela Rasmussen, a virologist at Columbia University's Mailman School of Public Health in New York City. "When it comes downwards to it, we still don't know a lot."

Minding human minds

Questions well-nigh masks go beyond biological science, epidemiology and physics. Human being behaviour is core to how well masks work in the real world. "I don't desire someone who is infected in a crowded area being confident while wearing ane of these cloth coverings," says Michael Osterholm, director of the Center for Infectious disease Research and Policy at the University of Minnesota in Minneapolis.

U.s. baseball players wore masks while playing during the 1918 influenza epidemic. Credit: Underwood And Underwood/LIFE Images Drove/Getty

Peradventure fortunately, some evidence¹² suggests that donning a face mask might bulldoze the wearer and those around them to adhere better to other measures, such as social distancing. The masks remind them of shared responsibility, perhaps. But that requires that people wear them.

Across the United States, mask apply has held steady effectually 50% since late July. This is a substantial increment from the 20% usage seen in March and Apr, co-ordinate to data from the Institute for Health Metrics and Evaluation at the University of Washington in Seattle (come across become.nature.com/30n6kxv). The institute's models also predicted that, every bit of 23 September, increasing U.s.a. mask use to 95% — a level observed in Singapore and some other countries — could save nearly 100,000 lives in the period up to i January 2021.

"In that location'southward a lot more nosotros would like to know," says Vos, who contributed to the analysis. "But given that it is such a uncomplicated, depression-cost intervention with potentially such a large impact, who would not want to use it?"

Further disruptive the public are controversial studies and mixed letters. 1 study^xiii in April found masks to exist ineffective, but was retracted in July. Another, published in June^xiv, supported the use of masks earlier dozens of scientists wrote a alphabetic character attacking its methods (meet get.nature.com/3jpvxpt). The authors are pushing back against calls for a retraction. Meanwhile, the World Health System (WHO) and the Usa Centers for Illness Control and Prevention (CDC) initially refrained from recommending widespread mask usage, in function considering of some hesitancy about depleting supplies for health-intendance workers. In April, the CDC recommended that masks exist worn when concrete distancing isn't an option; the WHO followed accommodate in June.

There's been a lack of consistency among political leaders, as well. US President Donald Trump voiced support for masks, but rarely wore one. He even ridiculed political rival Joe Biden for consistently using a mask — only days before Trump himself tested positive for the coronavirus, on 2 October. Other world leaders, including the president and prime government minister of Slovakia, Zuzana Čaputová and Igor Matovič, sported masks early in the pandemic, reportedly to set an example for their country.

Kingdom of denmark was one of the last nations to mandate face up masks — requiring their use on public transport from 22 August. It has maintained generally practiced control of the virus through early on stay-at-home orders, testing and contact tracing. It is also at the forefront of COVID-19 face-mask research, in the course of two large, randomly controlled trials. A research group in Denmark enrolled some 6,000 participants, asking half to apply surgical face masks when going to a workplace. Although the study is completed, Thomas Benfield, a clinical researcher at the University of Copenhagen and 1 of the principal investigators on the trial, says that his squad is not ready to share any results.

Benn's squad, working independently of Benfield's group, is in the process of enrolling around forty,000 people in Guinea-bissau, randomly selecting half of the households to receive bilayer material masks — 2 for each family fellow member aged 10 or over. The team will then follow anybody over several months to compare rates of mask apply with rates of COVID-similar illness. She notes that each household will receive advice on how to protect themselves from COVID-19 — except that those in the control group will not go information on the use of masks. The squad expects to consummate enrolment in November.

Several scientists say that they are excited to see the results. But others worry that such experiments are wasteful and potentially exploit a vulnerable population. "If this was a gentler pathogen, information technology would exist cracking," says Eric Topol, director of the Scripps Research Translational Constitute in La Jolla, California. "You tin can't do randomized trials for everything — and you shouldn't." As clinical researchers are sometimes addicted of proverb, parachutes take never been tested in a randomized controlled trial, either.

But Benn defends her work, explaining that people in the control group will still benefit from information about COVID-xix, and they will go masks at the terminate of the written report. Given the claiming of manufacturing and distributing the masks, "under no circumstances", she says, could her squad have handed out enough for everyone at the study'southward outset. In fact, they had to scale dorsum their original plans to enrol 70,000 people. She is hopeful that the trial will provide some benefits for everyone involved. "But no one in the customs should be worse off than if we hadn't washed this trial," she says. The resulting data, she adds, should inform the global scientific contend.

For now, Osterholm, in Minnesota, wears a mask. Yet he laments the "lack of scientific rigour" that has so far been brought to the topic. "We criticize people all the time in the science world for making statements without any data," he says. "We're doing a lot of the aforementioned affair hither."

Nevertheless, virtually scientists are confident that they tin say something prescriptive about wearing masks. It's not the only solution, says Gandhi, "but I think information technology is a profoundly important colonnade of pandemic command". As Digard puts information technology: "Masks work, but they are non infallible. And, therefore, continue your distance."

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Source: https://www.nature.com/articles/d41586-020-02801-8

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