Scientific research shows that masks have saved many lives during the new coronavirus pandemic, but the debate about masks continues. How much evidence is necessary to be considered sufficient? This article is from the WeChat official account: Nature Nature Research (ID: Nature-Research) , original author: Lynne Peeples, original title: “The Effectiveness of Masks: What the Data Say”, the title picture comes from: Visual China

When a colleague from Denmark first suggested that the residents of Guinea-Bissau be distributed protective cloth masks to stop the spread of the new coronavirus, Christine Benn was not so sure.

“I said,’OK, that might be good, but the data on whether masks are effective is still limited.'” Benn, a global health researcher at the University of Southern Denmark in Copenhagen, said that she has been the world’s most Poor West African countries jointly lead the public health movement.

It was in March. By July, Benn and her team already knew how to provide some necessary data on masks, and hoped to help the residents of Guinea-Bissau. In a randomized controlled trial, they distributed thousands of locally produced cloth masks; this trial may be the world’s largest test of the effectiveness of masks to prevent the spread of new coronary pneumonia.

In this epidemic, masks have become a symbol that can be seen everywhere; the new crown virus has sickened 35 million people and killed more than 1 million. In hospitals and other medical institutions, the use of medical-grade masks has significantly reduced the spread of the new coronavirus. However, there are many types of masks used by the public, and the relevant data is messy and uneven, and many of them are hurriedly put together. Coupled with incompatible political rhetoric, including a US president expressing his contempt for the use of masks-just a few days before his own diagnosis of COVID-19. “Evidence talkers understand the evidence in different ways.” said Baruch Fischhoff, a psychologist at Carnegie Mellon University who specializes in public policy. “This is a reasonable way to confuse people.”

RequiredTo be clear, Scientific research supports the use of masks; the latest research shows that masks can save lives in different ways: Research shows that masks can reduce the chance of spreading and contracting the new coronavirus, and some studies suggest that masks may reduce infection The severity of the disease afterwards.

However, how effective the mask is or when to use the mask is a very complicated question. There are many types of masks, which are used in different environments. Whether people are willing to wear it, or whether to wear it correctly, there are problems. Even what kind of research can provide clear evidence that they are effective is difficult to answer.

“What kind of evidence is good?” Fischhoff asked, “This is a key question.”

Beyond the gold standard

At the beginning of the outbreak, medical experts lacked conclusive evidence to prove how the new crown virus spread, and they did not know enough about masks to make strong public health advice on masks.

The standard mask used in the medical scene is the N95 mask, which is designed to protect the wearer by filtering out 95% of suspended particles of 0.3 microns and above. As the epidemic intensified, these masks soon fell short of demand. This leads to a controversial question: Is it useful for the public to wear basic surgical masks or cloth masks? If the answer is yes, under what circumstances should I wear it? “Those were originally the questions we (to be clear) in clinical trials.” Johns Hopkins University Medicine Kate Grabowski, an infectious disease epidemiologist at the hospital, said, “But we don’t have time to do this.”

Therefore, scientists can only rely on observational research and laboratory research, as well as indirect evidence from other infectious diseases. “If you look at any article-it’s not the final word. However, on the whole, I believe masks are useful.” Grabowski said.

In June, news that two hair stylists in Missouri, USA tested positive for COVID-19[1] , So that people’s confidence in masks has greatly increased. Both wear double-layer cotton masks or surgical masks at work. Although they passed the virus to family members, theirCustomers seem to be spared (It is reported that more than half of customers have rejected free testing).

Other hints of effectiveness appeared at mass gatherings. In the “Black Lives Matter” (Black Lives Matter) protests in many places in the United States, most of the protesters wore masks. These activities do not seem to have caused the peak of infection[2], but the virus is In late June, it spread widely during a summer camp in Georgia. Children who participated in the summer camp were not required to wear masks.[3] .

There are many points to note here: the protests are outdoors, so the risk of spreading new crown pneumonia is low, and campers will stay together at night. Moreover, since many non-protesters stay at home during the rally, this may reduce the spread of the virus in the community. Nonetheless, the anecdotal evidence “pieces the whole picture together,” said Theo Vos, a health policy researcher at the University of Washington.

Direct evidence comes from more rigorous analysis. According to a preprinted study published in early August [4] (not yet peer reviewed), and Compared with other regions, where masks become standard or where the government recommends the use of masks, the weekly increase in per capita mortality is a quarter of the former. The researchers investigated 200 countries, including Mongolia, which promoted the use of masks in January, and as of May, no deaths related to new coronary pneumonia have been recorded.

Another study [5] examined the use of masks in April and May by states in the United States Effect. Researchers estimate that these regulations reduce the increase in new coronary pneumonia cases by up to 2 percentage points per day. They carefully proposed that after considering other mitigation measures (such as maintaining physical distance), This mandatory regulation may have avoided as many as 450,000 infections.

Jeremy Howard, a researcher at the University of San Francisco, said: “You don’t need to do too much calculations to see that this is definitely a good way.” His team evaluated in a widely circulated preprint article Evidence of wearing a mask[6].

However, this kind of research does rely on the assumption that mask regulations are enforced and people wear masks correctly. In addition, the use of masks often coincides with other changes, such as restrictions on gatherings. With the lifting of restrictions, further observational studies may begin to separate the effects of masks from the effects of other interventions, Grabowski suggests. “That’s easier to see what’s doing.” She said.

Although scientists cannot control many confounding variables in the human population, they can be in animal research. Under the leadership of Yuan Guoyong, a microbiologist at the University of Hong Kong, the researchers placed the infected hamsters and healthy hamsters in adjacent cages, and separated some of them with surgical masks. According to a paper [7] published in May, about one-third of them are The second uninfected hamster was infected with the new coronavirus. However, only about 25% of hamsters protected by masks are infected, and those infected hamsters have less symptoms than “neighbors” without masks (based on clinical scores and organization Change to measure).

The above research results confirm the emerging consensus that using masks can protect both the wearer and others. This work also points to another point of view that may change the rules of the game: “Masks can not only protect you from infection, but also help you reduce symptoms.”Monica Gandhi, an infectious disease doctor at the University of California, San Francisco Say.

A paper co-authored by Gandhi was published in late July[8] , indicating that wearing a mask will reduce the amount of virus that the wearer may be infected, thereby making infectionsThe symptoms are mild and even asymptomatic. She believes that a greater amount of virus will lead to more severe inflammation.

She and her colleagues are currently analyzing the hospitalization rate of new coronary pneumonia in 1,000 counties in the United States before and after the introduction of mask regulations to determine whether the severity of the disease has decreased after the introduction of public mask wearing guidelines.

Paul Digard, a virologist at the University of Edinburgh in the United Kingdom, said that exposure to more viruses will lead to more serious infections. This idea is “absolutely reasonable.” “This is another argument about masks.” He didn’t. Participate in the above research.

Gandhi proposed another possible benefit: If more people suffer from mild illness, it may help strengthen herd immunity without increasing the burden of severe illness and death. “Whether we are waiting for a vaccine, will promoting an increase in the asymptomatic infection rate be beneficial to herd immunity?” she asked.

Rediscussing the transmission channels

The controversy surrounding masks is closely related to another divergent question: How does the virus spread infection in the air?

When people breathe or talk, sneeze or cough, small liquid particles will be ejected. Some are large or even visible and are called droplets; some are microscopic and are classified as aerosols. Viruses, including the new coronavirus, will “catching up” with these particles; their size determines their behavior.

The droplets will be ejected through the air and fall on the eyes, nose or mouth of people around, causing infection. But under the action of gravity, they will fall down quickly. In contrast, aerosols can float in the air for several minutes to several hours and spread like cigarette smoke in an unventilated room.

The time-lapse image shows how droplets from a cough spread from an N95 mask wearer. The N95 mask here has a valve that can Expel exhaled air. Source: S. Verma et al./Phys. Fluids.

What does this mean for the ability of masks to hinder the spread of COVID-19? The diameter of the virus itself is only about 0.1 microns. However, because the virus does not leave the body on its own, masks do not need to be able to block such small particles to be effective. More closely related are the droplets and aerosols that transmit pathogens. Their diameters range from about 0.2 microns to hundreds of microns. (The average diameter of a human hair About 80 microns). Jose-Luis Jimenez, an environmental chemist at the University of Colorado at Boulder, said that most of them are 1-10 microns in diameter and can stay in the air for a long time. “That’s the problem.”

Scientists are still not sure which particle size is most important to the spread of new coronary pneumonia. Some people cannot even agree on the criteria for defining aerosols. For the same reason, scientists still don’t know what the main form of influenza transmission is-although they have been studied for longer.

Many people believe that asymptomatic transmission is the main driver of the new coronary pneumonia pandemic. If this is the case, it means that the virus usually does not rely on coughing or sneezing. According to this reasoning, aerosol may prove to be the most important carrier of transmission. So, which masks can block aerosols is worth studying.

Material is the key

Even the tightly-fitting N95 masks cannot reach the full 95% level in actual use. In fact, they can only filter out about 90% of inhaled aerosols—the minimum is 0.3 microns. According to unpublished research, N95 masks without an exhalation valve (can expel unfiltered exhaled air) can only block similar proportions Exhale the aerosol. Kevin Fennelly, a lung scientist at the National Heart, Lung and Blood Institute in the United States, said that people know much less about surgical masks and cloth masks than this.

An international research team estimated in a review of observational research [9] that surgical masks The protective effect of the same cloth mask on the wearer is67%.

Linsey Marr, an environmental engineer at Virginia Tech, and her colleagues found that even a cotton T-shirt can block half of the inhaled aerosols and nearly 80% of the exhaled aerosols(2 microns in diameter). She said that once the aerosol reaches 4 to 5 microns, almost any fabric can block more than 80% of the inhaled/exhaled aerosol. The results of their work have not yet been published.

She added that multilayer fabrics are more effective, and the denser the weave, the better. Another study [10] found that masks with multiple layers of different materials-such as cotton Hesi-can capture aerosols more effectively than a single-material mask.

Benn worked with Danish engineers at her university to test their double-layer cloth mask design using the same standards as medical-grade ventilators, and found that their masks can only block 11%-19% of the iconic 0.3 micron Aerosol. But according to Marr and Jimenez, since most transmission may be through particles with a diameter of at least 1 micron, the actual difference in effectiveness between N95 and other masks may not be great.

In August, Eric Westman, a clinical researcher at Duke University School of Medicine, co-authored and published a study [11], showing a method to test the effectiveness of masks. His team used lasers and smartphone cameras to compare the effectiveness of 14 different cloth masks and surgical masks to stop droplets when people talk. He said: “I’m relieved that many of the masks we use are indeed effective.” But the thin polyester-spandex snood—a kind of elastic scarf that can be pulled over the nose and mouth—seems to reduce the droplets that are released. size. “That might be worse than wearing nothing,” Westman said.

Some scientists suggest not to interpret this discovery too much, because it is based only on the situation when a person is speaking. Like other scientists, Marr and her team responded with experiments and found that most of the large droplets were blocked by the collar. Marr said that she is writing her results and preparing for publication.

“There is a lot of information, but putting all the evidence together can feel confusing.” Columbia University Mel