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Editor’s Note: This story was updated July 11 to include information on why valuable masks don’t block exhaled droplets.

As states reopen stay-at-home orders, many states, including California, are now requiring people to wear face coverings in most public spaces to reduce the spread of COVID-19.

Both the Centers for Disease Control and Prevention (CDC) and the World Health Organization now recommend cloth masks for the general public, but earlier during the pandemic, both organizations recommended the opposite. These changed guidelines may have caused confusion among the public about the use of masks.

But health experts say the evidence is clear that masks can help stop the spread of COVID-19 and the more people wear them, the better.

We spoke with epidemiologist George Rutherford, MD of UC San Francisco and infectious disease expert Peter Chin-Hong, MD, about the CDC’s reversal to mask wearing, the current science of How masks work and what to consider when choosing a mask.

Why did the CDC change its guidance on wearing masks?

The CDC’s initial guidance was based in part on what was thought to be low infection rates earlier in the pandemic, Chin-Hong said.

“So, of course, you’re preaching that juice isn’t really worth forcing the entire population to wear masks in the first place – but that’s really a reflection of not having enough testing,” he said. speak. “We were getting a false sense of security.”

Rutherford was more blunt. The legitimate concern that the limited supply of surgical masks and N95 respirators should be spared for healthcare workers will not prevent a more nuanced message about the benefits of mask wearing. “We should have told people to wear cloth masks to protect against bats,” he said.

Another factor “is that culturally, the United States is not really prepared for mask-wearing,” unlike some countries in Asia, where the practice is more common, Chin-Hong said. Even now, some Americans are choosing to ignore CDC guidance and local mask regulations, a hesitation that Chin-Hong says is “stupid.”

What might ultimately convince the CDC to change its guidance in favor of masks is the rising incidence of the disease and a clearer understanding that both pre-symptomatic and asymptomatic transmission is possible – even popular. Studies have found that viral load peaks in the days before symptoms begin, and simply speaking is enough to clear viral droplets.

“I think the biggest thing with COVID right now shaping all these guidelines on masks is that we can’t tell who has been infected,” Chin-Hong said. “You can’t look at a crowd and say, oh, that person should wear a mask. There are a lot of infections that have no symptoms, so everyone has to wear a mask.”

What evidence do we have that wearing a mask is effective in preventing COVID-19?

There is some evidence to support the effectiveness of masks.

One type of evidence comes from laboratory studies of respiratory droplets and the ability of different types of masks to block them. A test using high-speed video showed that hundreds of droplets ranging in size from 20 to 500 micrometers were produced when a simple phrase was said, but nearly all of these droplets were blocked when the mouth was covered by a mask. damp towel. Another study of people with the flu or the common cold found that wearing a surgical mask significantly reduced the amount of respiratory virus emitted in droplets and aerosols.

But the strongest evidence in favor of masks comes from studies of real-world scenarios. “The most important thing is the epidemiological data,” says Rutherford. Because it would be unethical to designate people not to wear masks during a pandemic, epidemiological evidence is derived from so-called “nature’s experiments”.

A recent study published in Health issues, for example, compares COVID-19 growth rates before and after mandatory mask wearing in 15 states and the District of Columbia. It found that masking tasks resulted in a slowing of daily COVID-19 growth, which became more apparent over time. The first five days after the mandate, daily growth slowed by 0.9 percentage points compared to five days before the mandate; in three weeks, the daily growth rate slowed by 2 percentage points.

Another study looked at coronavirus deaths across 198 countries and found that those with a cultural norm or government policy in favor of mask wearing had lower mortality rates.

Chin-Hong and Rutherford say two convincing case reports also suggest masks can prevent transmission in high-risk situations. In one case, a man flew from China to Toronto and later tested positive for COVID-19. He developed a dry cough and wore a mask on the flight, and all 25 people closest to him on the flight tested negative for COVID-19. In another case, in late May, two hairstylists in Missouri had close contact with 140 clients while sick with COVID-19. Everyone was wearing a mask and no customer tested positive.

Do masks protect the people who wear them or those around them?

“I think there is enough evidence to say that it is best for people with COVID-19 to protect them from giving COVID-19 to others, but you will still benefit from wearing a mask. page otherwise ‘no COVID-19’,” Chin-Hong said.

Masks can be more effective as “source control” because they can prevent large droplets from evaporating out into smaller droplets that can travel further.

Another factor to remember, Rutherford notes, is that you can still get the virus through the inner membrane of your eye, a risk that covering your face doesn’t eliminate.

How many people need to wear masks to reduce transmission in the community?

“What you want is 100 per cent of people to wear masks, but you’re going to tackle 80 per cent,” says Rutherford. In one simulation, researchers predicted that 80% of the population wearing masks would do more to reduce the spread of COVID-19 than strict lockdowns.

The latest forecast from the Institute for Health Metrics and Evaluation shows that 33,000 deaths by October 1 could be avoided if 95% of people wore masks in public.

Even if you live in a community with few people wearing masks, you’ll still reduce your own chance of contracting the virus by wearing a mask, Chin-Hong and Rutherford said.

Does the type of mask matter?

Studies have compared different mask materials, but for the general public, the most important thing may be comfort. Chin-Hong says the best mask is one you can wear comfortably and consistently. N95 respirators are only needed in medical situations such as intubation. Surgical masks often provide better protection than cloth masks, and some people find them lighter and more comfortable to wear.

The bottom line is that any mask that covers the nose and mouth is beneficial.

“The concept is risk reduction rather than absolute prevention,” says Chin-Hong. “You shouldn’t swing your arms if you think the mask isn’t 100 percent effective. That is silly. No one takes cholesterol-lowering drugs because they will prevent a heart attack 100 percent, but you are reducing your risk significantly. “

However, both Rutherford and Chin-Hong warn against using N95 respirators with valves (commonly used in construction to prevent inhalation of dust) as they do not protect those around you. These check valves close when the wearer inhales, but open when the wearer exhales, allowing unfiltered air and water droplets to escape. Chin-Hong said that anyone wearing a mask with a frill will need to wear a surgical mask or cloth over it. “Also, just wearing a mask has no value,” he said.

San Francisco has mandated that valved masks do not comply with the city’s face covering order.

If we are practicing social distancing, do we need to wear masks?

One mnemonic that Chin-Hong likes is the “Three Ws to ward off COVID-19:” wear a mask, wash your hands, and observe your distance.

“But of these three, the most important is wearing a mask,” he said. Compared to wearing a mask, cleaning your iPhone or cleaning the grocery store “are just distractions.” “There is little evidence that aerosols (contaminated surfaces) are the main source of transmission, while there is plenty of evidence of transmission via inhaled droplets,” Chin-Hong said. know.

“You should always wear a mask and keep social distance,” says Rutherford. “I would hesitate to try to analyze it. But, yes, I think wearing a mask is more important.”

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