Are masks effective against COVVI-19?

When her colleagues first suggested distributing cloth face masks to people in Guinea-Bissau to stem the spread of Covid-19, Christine Benn wasn't sure it would help.

“Yes, it may be good, but there is little data on the real effectiveness of masks,” said this health researcher at the University of Copenhagen, who has led for decades of public health campaigns in this West African country, one of the poorest in the world.

It was in March. But in July Christine Benn and her team found a way to obtain reliable data on the effectiveness of masks against the spread of Covid-19. They distributed thousands of locally made cloth masks to residents of Guinea-Bissau in a randomized controlled trial, the largest global experiment on this topic to date.

Masks are the ubiquitous symbol of a pandemic that, as of mid-October, has affected nearly 40 million people and killed more than 1.1 million. In hospitals and other health establishments, the use of specific masks (the famous FFP2 masks, in Europe) clearly reduces the transmission of the coronavirus. But for the various kinds of masks used by the general public (so-called “surgical” masks, cloth masks, etc.), the data is disparate, fragmented and often collected without great rigor. Add to that a changing political discourse, even one that denigrates the use of masks, as Donald Trump does, and the confusion becomes general. "The problem is that people interpret the data differently," says Baruch Fischhoff, a psychologist at Carnegie-Mellon University in Pittsburgh, Pennsylvania, who specializes in public policy. "It's legitimately confusing. »

To be clear, science supports the use of masks. Recent studies indicate that they are beneficial in various ways: they reduce the risk of transmitting or catching the coronavirus, and even could reduce the severity of infection in the event of contamination.

But it is more difficult to determine with certainty whether a particular mask works well or when it is most useful to wear it. There are many types of masks, used in various environments. It is not clear to what extent people agree to wear them, when exactly they wear them and if they put them on correctly. Even the question of what type of study would provide definitive proof of their effectiveness has no obvious answer. "What should be the level of proof? Asks Baruch Fischhoff. This is a vital question. »

Beyond the gold standard

At the start of the pandemic, experts did not know precisely how SARS-CoV-2 spread, and did not have enough evidence to make firm recommendations on the use of masks.

The mask used in healthcare facilities in Europe is the FFP2 mask (its equivalent is the N95 in the United States and the KN95 in China). It is designed to filter 95% of particles 0.3 microns or larger. As the pandemic escalated, these masks quickly found themselves in short supply. This raised a now contentious question: should citizens wear standard surgical masks or cloth masks? And if so, under what conditions? "Normally, these questions are answered with clinical trials," says Kate Grabowski, an epidemiologist at Johns Hopkins University School of Medicine in Baltimore, Maryland. "But at the start of the pandemic, we didn't have the time for that. »

The scientists therefore relied on observations and laboratory studies. Indirect evidence also comes from the use of masks in other respiratory infectious diseases. "If you look at each of these studies individually, it's not compelling evidence. But, taken as a whole, these clues seem convincing to me that masks are effective against Covid-19, ”said Kate Grabowski.

This confidence in the effectiveness of masks grew in June with the case of two Missouri hairdressers who tested positive for Covid-19. Both wore a double-layered cotton mask or a surgical mask when working. And although they transmitted the virus to members of their household, their clients seem to have been spared (even if half would have refused the free tests). Other clues appeared during the mass gatherings. At Black Lives Matter protests in US cities, most attendees wore masks. These demonstrations do not appear to have caused spikes in infection, as the virus spread in late June at a summer camp in Georgia, where children and teenagers were not required to wear a mask. However, there are many reservations: the demonstrations took place in the open air, which presents a lower risk of the spread of Covid-19, while the young people of the camp shared bungalows at night, for example. And since many people stayed at home during the protests, this could have reduced the transmission of the virus in the community. Still, anecdotal evidence like this “paints a picture,” says Theo Vos, a health policy researcher at the University of Washington in Seattle.

More rigorous analyzes have provided direct evidence. A study prepublished in early August (but not yet peer-reviewed) found that the weekly increase in per capita mortality was four times lower in places where masks were the norm or mandated by authorities, compared to other regions. The researchers looked at 200 countries. In Mongolia, which adopted the wearing of masks in January, in particular, no deaths linked to Covid-19 were recorded in May. Another study looked at the effects of measures taken by several US states that made mask-wearing mandatory in certain settings in April and May. The researchers estimated that these measures reduced the rate of increase in Covid-19 cases by 2 percentage points per day. They suggest – cautiously – that these measures would have prevented up to 200,000 cases in the United States.

“You don’t have to do fancy math to figure out that masks are obviously a good idea,” says Jeremy Howard, a researcher at the University of California, San Francisco. With colleagues, he reviewed the various evidence for the effectiveness of wearing face masks in a widely circulated preprint article.

But such studies rely on assumptions that recommendations for mask-wearing are being followed by the population, and that people are wearing their masks correctly. In addition, wearing a mask often goes hand in hand with other measures, such as distancing or limiting gatherings. When the restrictions are lifted, additional observational studies may succeed in distinguishing the impact of masks from that of other measures, suggests Kate Grabowski. “It will be easier to see the effect of each factor. »

Many confounding variables cannot be controlled in studies of human populations, but this is not the case in animal studies. The team of microbiologist Yuen Kwok-yung, from the University of Hong Kong, for example, housed infected hamsters and healthy hamsters in neighboring cages, separated or not by a partition of the same material as the surgical masks. The team's May paper reports that without this barrier, about two-thirds of healthy animals were infected with SARS-CoV-2, while only about a quarter of hamsters protected by mask partitions were infected. . In addition, those who were were less ill than their unprotected neighbors (as measured by clinical scores and tissue damage).

These findings substantiate the consensus that has emerged that wearing a mask protects both the wearer and others. Research also highlights another potentially game-changing idea: “The mask can not only protect you from infection, but also reduce the severity of Covid-19,” says Monica Gandhi, an infectious disease doctor at the University of California at San Francisco.

Monica Gandhi is the co-author of an article published at the end of July which suggests that the mask reduces the amount of viral particles that its wearer can receive, which, if infected, leads to a less severe form, even asymptomatic, of the disease. According to the researcher, a higher viral load would indeed lead to a more aggressive inflammatory reaction.

Monica Gandhi and her colleagues are currently analyzing Covid-19 hospitalization rates before and after the introduction of mask-wearing measures in 1,000 US counties to determine whether the severity of the disease decreased after the generalization of masks.

The idea that exposure to more virus leads to more severe infection makes “absolute sense,” says Paul Digard, a virologist at the University of Edinburgh, UK, who has no did not participate in the research. “This is another argument in favor of masks. »

Monica Gandhi suggests another possible benefit: if more people get mild cases, it could help boost the population's herd immunity without increasing the death toll or the strain of severe forms on people. health systems. “While waiting for a vaccine, could the increase in the rate of asymptomatic infections improve the overall immunity of the population? »

Back to ballistics

The mask debate is closely tied to another hotly debated question: how does the virus move through the air and spread infection ?

As soon as a person breathes or speaks, sneezes or coughs, a fine spray of liquid particles takes flight. Some are large and even visible, we speak of droplets; others are microscopic, and are classified as aerosols. Viruses latch onto these particles, the size of which dictates behavior.

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The droplets can be thrown through the air and land on the eyes, nose or mouth of a nearby person and cause an infection. But gravity quickly causes them to fall back to the ground. Aerosols, on the other hand, can float in the air for several minutes or even hours and spread through an unventilated room much like cigarette smoke.

What does this imply for the ability of masks to hinder the transmission of Covid-19? The virus itself is only about 0.1 micrometers in diameter. But since viruses don't leave the body on their own, a mask doesn't need to block such small particles to be effective. The vectors that should be blocked are droplets and aerosols that carry pathogens. Their diameter can vary between 0.2 micrometers and hundreds of micrometers, with a typical size of 1 to 10 micrometers. These aerosols can remain suspended in the air for a long time, explains José-Luis Jiménez, a chemist at the University of Colorado at Boulder.

Scientists still do not know which particle size is most important in the transmission of Covid-19. They do not even all agree on the threshold that defines aerosols. And for the same reasons, we still do not know with certainty what is the main mode of transmission of the flu, which has been studied for much longer.

Many epidemiologists believe that transmission from asymptomatic people is driving the Covid-19 pandemic, suggesting that SARS-CoV-2 is not only spread through sneezes and coughs. According to this reasoning, aerosols would be the main vector of transmission. It is therefore essential to determine whether and which masks can stop these aerosols.

It's all in the fabric

Under real-world use, even tight-fitting N95 masks have slightly lower efficiency than the advertised filtration rate of 95% of aerosols 0.3 micrometers and larger : they are closer to 90%. And, according to yet unpublished studies, N95 masks without an exhalation valve – which expel exhaled air without filtering it – block a similar proportion of exhaled aerosols. Much less is known about surgical and cloth masks, says Kevin Fennelly, a pulmonologist at the Heart, Lung and Blood Institute in Bethesda, Maryland.

A systematic review of 172 observational studies conducted in 16 countries by an international team concluded that surgical masks and comparable cloth masks reduce the risk of becoming infected by 67%.

In an as yet unpublished study, Linsey Marr, an environmental engineer at Virginia Tech University, and her colleagues found that even a cotton T-shirt can block half of inhaled 2-micron aerosols and nearly 80 % of those expired. And above 4 to 5 micrometers, almost any fabric blocks more than 80% of aerosols in both directions, she says.

Multiple layers of fabric are more effective than one, and the tighter the weave, the better. Another study found that masks with multiple layers of different materials — cotton and silk, for example — stop aerosols more effectively than those made from a single material.

Christine Benn worked with engineers at her Danish university to test the design of their two-layer fabric mask using the same criteria as for N95 or similar masks. They found that their mask only blocked 11-19% of 0.3 micrometer aerosols. But since most contamination is likely via particles at least 1 micrometer in size, according to Linsey Marr and José-Luis Jiménez, the actual difference in effectiveness between the N95 mask and cloth masks might not be huge.

Along with his colleagues, Eric Westman, a clinician scientist at Duke University School of Medicine in Durham, North Carolina, proposed in a paper published in August a proof-of-principle of a simple method for test the effectiveness of the masks. Using a laser and the camera of a simple smartphone, his team visualized the amount of droplets expelled by a man who speaks while wearing face protection, for 14 different types of protection, from surgical masks to simple clothing to fabric masks. “I was reassured by the fact that most of the masks that people use work satisfactorily,” he explains, referring to cloth masks and surgical masks. But stretchy fabric chokers pulled over the mouth and nose instead seem to reduce the size of the droplets released, "which could be worse than wearing nothing at all," according to Eric Westman.

Some researchers, however, doubt this last claim, based on a single person. Linsey Marr and her team conducted their own experiments, and say they found that neckbands blocked most large droplets. The researcher is in the process of formatting her results for publication.

“There is a lot of information on the effectiveness of masks, but when you put all the studies next to each other, it is not easy to know what to think,” explains Angela Rasmussen, virologist at the Mailman School of Public Health at Columbia University, New York. “At the end of the day, we still don't know much. »

In people's minds

The question of the effectiveness of masks goes beyond biology, epidemiology and physics. Human behavior is a key parameter of this equation in the actual conditions of use of these facial protections. "An infected person shouldn't feel overconfident wearing one of these cloth masks in a busy place," says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota at Minneapolis.

Fortunately, a study on people's practice of social distancing recommendations, in pre-publication, suggests that wearing a mask encourages the wearer and those around them to better adhere to other measures, such as social distancing. Masks are perhaps a visible reminder of a shared responsibility. But for that, people still have to wear them.

According to data from the Institute for Health Metrics and Evaluation at the University of Washington in Seattle, USA, the proportion of people who report always wearing a mask in public ranges from 60 % and 70% since the end of July. This is a substantial increase from the 10-20% mask adoption seen in March and April. The Institute's models also project that increasing the mask usage rate to 95% — a level seen in Singapore and some other countries — could save nearly 100,000 lives in the United States by the end of the day. end of the year.

“There are a lot of things we would like to know more about the effectiveness of masks,” says Theo Vos, who contributed to this analysis. "But given that it's such a simple, inexpensive gesture with such great potential impact, who wouldn't want to use it? »

Controversial studies and conflicting messages only confuse the public further. A study done in April that claimed masks are ineffective was retracted in July. Another, published in June, pointed to the central role of aerosols in contamination and the effectiveness of masks to protect against it, but dozens of researchers attacked its methodology and demanded its retraction, to which the authors of the study objected. Meanwhile, the World Health Organization (WHO) and the US Centers for disease control and prevention (CDC) initially declined to recommend widespread mask use, in part for fear of creating a mask shortage. for health workers. In April, the CDC ended up recommending wearing a mask when physical distancing isn't an option, and the WHO did the same in June.

Political leaders have also shown a lack of consistency. After several months of denial, Donald Trump finally agreed to recommend wearing a mask in July, but he rarely wore one… even after falling ill with Covid-19 on October 2. Conversely, other world leaders, such as Slovak President Zuzana Čaputová, wore masks from the start of the pandemic to set an example for their fellow citizens.

Denmark was one of the last countries to make it compulsory to wear a mask, which has only been required on public transport since August 22. The country has maintained good overall control of the virus through early containment measures, testing and contact tracing. It is also at the forefront of protective mask research, with two large randomized controlled trials. A research group in Denmark recruited some 6,000 participants, asking half of them to use surgical masks in the workplace, and the other not. The experiment is over, but according to Thomas Benfield, a clinical researcher at the University of Copenhagen and one of the lead authors, the results are not yet ready to be shared.

Christine Benn's team, which works independently of Thomas Benfield's, is enrolling around 40,000 people in Guinea-Bissau, randomly selecting half of the households that will receive two-layer fabric masks – two for each family member aged 10 or over. The team will then follow everyone for several months to compare mask usage rates with Covid-19 rates. Each household will receive advice on how to protect themselves against infection, except those in the control group who will not receive information on the use of masks. The team expects to complete participant recruitment in November.

Several scientists say they are excited to see the results of this study. But others fear the experiment is unnecessary and potentially endangers a vulnerable population. “If it was a mild virus, that would be very interesting,” says Eric Topol, director of the Scripps Research Translational Institute in La Jolla, California. “But you can't do randomized trials for everything – and you shouldn't. As clinicians like to recall, parachutes have never been tested in a randomized controlled trial…

Christine Benn nevertheless defends her trial, explaining that people in the control group will still benefit from information on Covid-19 and that they will receive masks at the end of the experiment. Given the challenge of manufacturing and distributing the masks, “under no circumstances,” she said, could her team have distributed enough for everyone at the start of the study. In fact, the researchers had to scale back their initial plans, which aimed to recruit 70,000 people. Christine Benn hopes this trial will benefit everyone involved. "No one in this community should be worse off than if we hadn't done this trial," she says. The resulting data, she adds, should fuel the global scientific debate.

Right now, in Minnesota, Michael Osterholm is wearing a mask. However, he deplores the “lack of scientific rigor” with which this subject has so far been treated. “In science, we have a habit of criticizing people who say things without having evidence. Yet that is what we often do these days. »

Nevertheless, the majority of scientists are convinced that we can recommend wearing masks. “It is not the only solution, admits Monica Gandhi, but I think it is a major pillar of control of the pandemic. As Paul Digard says: “Masks work, but they are not infallible. Therefore, keep your distance. »