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How Much Does Wearing a Mask Protect You?

serious woman mask

Nov. 19, 2020 – Yes, you should still wear a mask.

You may have heard of a new study out of Denmark that tested whether paper surgical face masks protect the people who wear them.

The study comes just a week after the CDC updated its guidance on masks to say they aren’t just for the benefit of others, but they also help keep people who wear them from getting sick.

So this new study -- which found that these kinds of masks don’t appear to offer a big benefit to the wearer -- may feel a little bit like whiplash.

Like a lot of the science on COVID-19, this study has already been passed through the prism of the divided social moment in the U.S., with people interpreting its findings depending on their political leanings.

“Mask wearing doesn’t do a damn thing,” tweeted a conservative talk radio show host in response to the study.

But that’s not exactly what the study found. And the bottom line is that you should still wear a mask.

“Masks bring down the community viral load. There’s less getting out, and that means there is less for you to be exposed to,” says John Brooks, MD, a medical epidemiologist in the Division of HIV/AIDS Prevention at the CDC in Atlanta.

Brooks points to other studies, as well as the experience in other countries, as the basis of the agency’s recommendations.

In countries where mask use is high, case counts are low. That’s been true throughout history, too. Masks have long been deployed during outbreaks of infectious disease and have been shown to help control the spread of airborne germs.

But Brooks and other experts say the new study is important because it was well-done and it adds to what we know about the population-wide use of masks to control the spread of an airborne disease.

For some background, for several months now, conservative influencers have been pointing to the Danish mask study, known as DANMASK, as proof that the pandemic and the nondrug measures that public health experts have advised us to follow -- like mask-wearing and social distancing -- are unnecessary and too restrictive. The study wasn’t being published, they said, because the editors at scientific journals were afraid to make its findings public.

The study was published Wednesday -- and made available for free, as much of the research on COVID-19 has been, by a very respected scientific journal, the Annals of Internal Medicine.

The editor-in-chief of that journal, Christine Laine, MD, co-authored an editorial that’s running alongside the study.

Asked if she’d been afraid to publish the study, Laine said, “Afraid is probably not the right word.”

“We thought it was important to publish it, but we were concerned about the risk that people who did not carefully consider the question that this trial was able to answer would misinterpret it, either because they didn’t understand the study or purposefully to support their own beliefs about the effectiveness of masks.”

The study is important because it is the first of its kind. It’s the only time researchers have been able to test mask-wearing in a randomized controlled trial, which is the gold standard for scientific evidence.

Mask-wearing is considered protective now, but in Denmark in the spring, it wasn’t routine or even advised by public health authorities. If it had been, says lead study author Henning Bundgaard, PhD, a cardiologist at Copenhagen University Hospital, the study might not have been ethical, since it would have deprived one group of participants of a recognized layer of protection against a potentially deadly virus.

“So we had a golden opportunity to do the study during this period of time,” he says. Instead of potentially putting one group in harm’s way, they were actually putting some of their participants in a better position by having them wearing masks.

Back in April, researchers split 6,000 Danish citizens into two roughly equal groups. The first group was asked to wear a paper surgical mask anytime they went out in public for the next month. Those masks are about 98% effective at screening small particles, but they don’t fit snugly to the face the way N95 masks do. There are still gaps where unfiltered air can reach the nose and mouth.

The mask group watched a video explaining how to wear masks properly and got 50 free masks in the mail. The control group was assigned not to wear face masks. In fact, the researchers excluded people who wore masks for protection on the job. The control group was told to follow the advice of public health authorities. Before the study began, everyone was tested for antibodies to make sure they hadn’t already been infected with the virus. If they were positive, they were excluded from the trial.

The study was “powered” -- meaning that it included enough people -- to detect whether following the advice to wear a mask could cut the risk of catching COVID-19 by 50%, or half. It didn’t.

“That’s a big number in any clinical trial,” says F. Perry Wilson, MD, an associate professor at Yale University. Wilson writes the Methods Man blog, where he breaks down the findings of clinical trials. He was not involved in the current research.

In other words, the trial was designed to look for a big benefit for people who wear masks. It didn’t find that large benefit.

After 1 month, 42 people out of 2,392 in the group that wore masks, or 1.8%, developed a COVID-19 infection, compared to 53 people out of 2,470 in the group that didn’t wear masks, or 2.1%.

That’s a smaller benefit -- about a 16% reduction in infections, on average -- in people who said they wore their masks as they were told.

That result didn’t pass a test for statistical significance, though, meaning that it could have been due to chance.

“While it did not reach the 50% threshold that the researchers defined for significance, it's still a reduction and is consistent with results of other similar studies,” said Linsey Marr, PhD, a professor of civil and environmental engineering at Virginia Tech in Blacksburg, in a written response to questions about the study. Marr is testing the mechanical properties of different kinds of masks and learning exactly how they protect against the spread of the virus. She was not involved with the Danish study.

To get even more specific, the benefits or risks from masks reported in the study ranged anywhere from a 45% reduction in infection for mask wearers to a 20% increase in the risk of getting sick.

“There’s some meaningful number in that range that would lead us to either believe in masks or to abandon masks, and this study can’t tell us the difference between them,” says Wilson.

There are plenty of reasons not to abandon masks based on this one study.

First, while the study asked people to wear a mask, it didn’t monitor them to make sure that they actually did, or that they wore their masks properly.

The researchers tried to account for this by asking people how well they thought they did with compliance. More than half admitted that they didn’t wear their masks perfectly.

In some ways, though, that’s more practical, because that’s what we do in the U.S., too. We advise people to wear masks, Laine says.

“If you walk around any city in the U.S., some people are wearing masks over their noses and other ones are wearing them hanging off one ear,” she says, and that makes the study more of a practical real-world test.

The other important point about the study is that it didn’t test masks as a means of source control, or a way to keep people who are infected from passing the virus on to others.

“If you think about the history, you know the surgical mask was invented not to protect the surgeon, but to protect the patient from the surgeon; you know, coughing into the open surgical wounds and causing infection in the patient,” Laine says.

In that way, she says, masks are still a really important tool for community protection. The more people who wear one, the more we’re all protected.

Brooks says the CDC will not change its recommendations based on this research.

“Our recommendation remains the same, because we are emphasizing the utility of masking for community control of this,” he says.

But one thing the study should do is knock down any false sense of confidence that people may get when they have a mask on.

“It should give some pause to people who feel kind of invincible because they go out and spend $40 on some mask designed by NASA scientists or something,” Laine says, poking fun at some of the ads she says pop up in her Facebook feed. “They may feel like they can go to a crowded setting, you know, but, ‘I’m wearing a mask, so I’m fine.’ You’re probably not fine,” she says. “You are not invulnerable to infection.”

That’s why it’s important to not only wear a mask, but to also wash your hands properly, maintain at least 6 feet of space between you and another person, avoid large gatherings, etc.

Brooks agrees, but he says, “That’s not a reason not to use it. It has to be combined with other things because no one thing is perfect.”

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Sources

Henning Bundgaard, PhD, cardiologist, Copenhagen University Hospital, Copenhagen, Denmark.

F. Perry Wilson, MD, associate professor, Yale University; director, Clinical and Translational Research Accelerator, New Haven, CT.

Linsey Marr, PhD, professor of civil and environmental engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA.

Christine Laine, MD, editor-in-chief, Annals of Internal Medicine; clinical associate professor, internal medicine, Jefferson University Hospitals, Philadelphia.

John Brooks, MD, medical epidemiologist, Division of HIV/AIDS Prevention, CDC, Atlanta.

Annals of Internal Medicine: “Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers.”

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