What's the science behind masks? Do masks prevent or reduce COVID infection?

C. A. Bridges
Palm Beach Post
  • Aug. 27: Judge overturns Gov. DeSantis' ban on mask mandates in schools

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UPDATE: On August 27, a circuit court judge in Tallahassee overturned Gov. DeSantis' ban on mask mandates in schools, ruling that the governor overreached his authority, misinterpreted state law and ignored scientific evidence in issuing his order. DeSantis has said he will appeal the decision.

COVID numbers are rising again, with the more-contagious delta variant spreading rapidly among unvaccinated people. The CDC reversed course and urged even fully-vaccinated Americans to wear masks indoors again in areas of high COVID-19 transmission.

And the question arises again: do masks help?

There has been a changing and occasionally conflicting message on this topic from experts and the federal Centers for Disease Control and Prevention, so let's take a look at what we know now.

Short version: Yes. They do. A lot.

Repeated studies have shown that masks, when used properly, drastically reduce the amount of respiratory droplets you spread, and they reduce the amount you breathe in (but not as much). That slows down the spread of COVID, by reducing the droplets expelled by an infected person.

“We are not defenseless against COVID-19,” CDC Director Dr. Robert R. Redfield said in an editorial published in the Journal of the American Medical Association last July. “Cloth face coverings are one of the most powerful weapons we have to slow and stop the spread of the virus  particularly when used universally within a community setting. All Americans have a responsibility to protect themselves, their families, and their communities.”

But how do they work? And why are some people still confused about their effectiveness? Here's some answers.

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How do masks prevent the spread of COVID? Can't the virus just go right through cloth?

Coronavirus, which causes COVID-19, is spread mostly though exposure to respiratory droplets exhaled by infected people when they breathe, talk, cough, sneeze or sing. The louder or more violent those exhalations are, the farther those virus-infected droplets can go. If another person inhales these droplets or catches them in their mouth or nose, they may become infected.

Masks create a physical barrier to block those droplets and prevent them from spreading as far as they ordinarily would.

While the virus itself could certainly pass through cloth, the droplets that spread it can't. Cloth masks block most large droplets, and multi-layer cloth masks block a lot more, with reductions as high as 50-70%.

Masks are much more effective at preventing an infected person from spreading COVID than they are at keeping you safe from catching it, but it's still much better than not using a mask at all, especially if you're in an area with closely-packed infected people.

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How do we know that masks work to prevent the spread of COVID?

From experimental studies that test cloth masks' capacity to block both respiratory particles and inhalation of them, by examining specific cases of outbreaks, and by studying communities to see where COVID is spreading and where it isn't.

One study looked at the growth rate of COVID-19 before and after mask mandates in 15 states and the District of Columbia. In the first five days after the mandate, the daily growth rate slowed by 0.9% compared to the five days before the mandate. By three weeks, daily growth had slowed by 2%. The study estimated that as a result of the mandates, more than 200,000 COVID-19 cases were prevented during the period the study examined.

Early on there was a report in the CDC’s Morbidity and Mortality Weekly Report (MMWR) about two Missouri hairstylists who were infected and symptomatic and contined to see customers but wore masks. So did 98% of their clients. Among 139 clients, none of them displayed symptoms and the 67 who were tested were negative. 

When Kansas instituted a mask mandate last July but allowed counties to opt out, the counties with mandates saw the number of new COVID cases per capita fall by 6%. The numbers in counties without mask mandates in areas with mask mandates rose by 100%.

Another study examining 198 countries found lower death rates in countries with mask mandates compared to counties without.

similar study out of Germany published in December in the Proceedings of the National Academy of Sciences found that in the city of Jena, first in the country to mandate face masks last April, early introduction of mask wearing "resulted in a drop in newly registered COVID-19 cases of around 75% after 20 (days)."

There may be other reasons for the differences in infection growth — masks mandates may have accompanied lockdowns and increased social distancing, for example — but two CDC scientists described the overall evidence as compelling.

“Over the course of the pandemic, the scientific evidence and our understanding of masks have grown. The data we now have conclusively show that widespread use of masks is a very effective way to reduce the spread of COVID-19,” CDC spokeswoman Jasmine Reed wrote in an email to USA TODAY.

Fact check:Masks and vaccines are effective at combating COVID-19 spread

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Why should I wear a mask if I'm not sick?

The CDC estimates that 40-45% of infected people never develop symptoms. And even if you do, you will be the most contagious just before you display any symptoms and for a few days afterward.

At least half of all new infections come from people who are probably unaware they're infectious, the CDC says.

Didn't the CDC originally say not to use masks?

Yes, they did, for several reasons.

First, experts weren't sure yet how the disease spread. Initially it was assumed it came from infected people with symptoms, coughing and sneezing.

There was also the concern that the U.S. couldn't make personal protection equipment (PPE) fast enough and that panicked Americans would buy up all the stock, leaving health care workers on the front lines of the disease vulnerable.

“The word that we got was that we were struggling to make sure we get personal protective equipment, including masks, for the health care workers, so the initial recommendation was: Don’t put masks on, because we’re going to be taking them away from health care workers,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said. “That understandably got interpreted as, we didn’t think masks were of any benefit.”

Fauci actually told a Senate committee last March masks were unnecessary "because right now, there isn't anything going around right now in the community, certainly not coronavirus, that is calling for the broad use of masks."

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In late February, then-U.S. Surgeon General Jerome Adams even tweeted, “Seriously people — STOP BUYING MASKS.” He followed that up with public statements against masks and his concern that wearing them might do more harm than good, giving people a false sense of security that might result in less attention to hand-washing and social distance.

"And stay safe by washing your hands, by covering your cough, by staying home if you're sick. Masks do not work for the general public and preventing them from getting coronavirus," he told Margaret Brennan on CBS News' "Face the Nation."

Why did the CDC change its stance and tell people to start wearing masks?

"I was saying that then because of everything we knew about coronaviruses before that point told us that people were not likely to spread when they were asymptomatic," Adams said later. "So the science at the time suggested that there was not a high degree of asymptomatic spread.

"We learned more," he said.

As the virus spread to every state in the U.S. and every country in the world, it became obvious that people could contract and spread the virus without showing symptoms. Also, supplies of masks increased as the U.S. imported them from around the world, companies began cranking them out, and volunteers across the country began sewing them by the millions to donate.

In early April the CDC reversed its recommendation and began urging people to wear masks in public. That was nearly a month after the World Health Organization labeled the COVID-19 outbreak as a pandemic. The CDC even included tips on making them yourself.

Over the summer, anecdotal evidence began piling up: masks help slow the spread.

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Do vaccinated people need to wear masks?

According to the CDC's latest guidelines, updated July 27, yes.

In March, the CDC relaxed their recommendations for people vaccinated against COVID-19. Fully vaccinated people could now stop social distancing measures and mask-wearing indoors, with few exceptions.

"Anyone who is fully vaccinated can participate in indoor and outdoor activities, large or small, without wearing a mask or physical distancing,'' CDC Director Rochelle Walensky said during a White House briefing that day. "If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic.''

Why the change? At the time, the rates of new COVID cases, hospitalizations and deaths were dropping.

“Currently authorized vaccines in the United States are highly effective at protecting vaccinated people against symptomatic and severe COVID-19,” the CDC wrote in its May 13 announcement. “Additionally, a growing body of evidence suggests that fully vaccinated people are less likely to have asymptomatic infection or transmit SARS-CoV-2 to others.”

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Why did the CDC start recommending masks for everyone again?

The CDC updated their recommendations, urging fully vaccinated people to wear masks indoors again in areas with high transmission as COVID-19 cases continue to rise and vaccination rates wane, according to media reports. 

The CDC is also now recommending universal indoor masking for all teachers, staff, students and visitors inside schools from kindergarten to 12th grade, regardless of vaccination status, aligning closely with the American Academy of Pediatrics, which recommended this month that anyone over the age of 2 be required to wear a mask in school. 

"The delta variant is showing everyday its willingness to outsmart us and be an opportunist," CDC director Dr. Rochelle Walensky said during a Tuesday briefing. "In rare occasions some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others ... This new science is worrisome and unfortunately warrants an update to our recommendations."

This recommendation also brings the CDC in line with many local health departments that had already started reimposing mask mandates because of rising numbers of COVID cases.

“This gives more of an opportunity for local health departments to not look as though they’re doing something different than what the CDC is suggesting,” said Dr. William Schaffner, a professor and infectious disease expert at the Vanderbilt University School of Medicine in Nashville, Tennessee.

Daniel Salmon, director of the Institute for Vaccine Safety at Johns Hopkins Bloomberg School of Public Health called the CDC's relaxation of mask recommendations “one of the biggest missed opportunities we had."

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"Where I live, everyone just stopped wearing masks and social distancing," Salmon said. "They didn’t follow the CDC recommendations, they just thought, ‘Oh, I don’t have to wear a mask anymore.’”

Do you need to wear two masks?

“So, if you have a physical covering with one layer, you put another layer on, it just makes common sense that it likely would be more effective," Fauci told "NBC News’ TODAY" in January. “That’s the reason why you see people either double masking or doing a version of an N95.”

A study published in the peer-reviewed journal Matter in July found that wearing two snug face coverings could protect against up to 90% of particles by adding an extra layer of protection and making the mask fit more tightly around the face.

Interest in this picked up around the same time more-contagious variants started emerging. But double-masking hasn't really caught on and the CDC has not recommended it.

“The idea of double masking just simply means that you’re paying attention to wearing a better mask than you might have been before,” Emory University infectious disease expert Dr. Colleen Kraft said. “If people would just wear masks in general, we would actually be fine against the variants.”

Double-masking:Protect yourself from COVID variants

Aren't masks dangerous? Does wearing a mask reduce your oxygen levels?

One of the claims by skeptics against masks is that they cause oxygen deprivation in the brain. A viral Facebook post transcribed a video message from neurologist Margareta Griesz-Brisson where she said masks can cause neurological damage, especially in children. The video was removed by YouTube for violating its terms of service.

However, a study of people wearing surgical masks while walking around showed no significant change in carbon dioxide in their breath or oxygen in their blood comparted to walking without a mask.

And medical experts disputed those claims, sometimes to great lengths.

The University of California-San Diego Health office posted a video on its YouTube page in which one of its demonstrators wore four masks at once and saw her oxygen level remain at 98%, according to a pulse oximeter, which measures oxygen saturation levels in the blood.

When she wore six masks at the same time, her oxygen levels dropped to 97%. Normal oxygen levels using a pulse oximeter range from 95%-100%, experts say.

There have been many claims about health hazards from wearing masks, but crtiics have said the studies were flawed..

Fact check:Experts say face masks don't cause oxygen deprivation, neurological damage

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Are masks required anywhere?

According to the CDC, "Masks are required on planes, buses, trains, and other forms of public transportation traveling into, within, or out of the United States and in U.S. transportation hubs such as airports and stations. Travelers are not required to wear a mask in outdoor areas of a conveyance (like on a ferry or the top deck of a bus). CDC recommends that travelers who are not fully vaccinated continue to wear a mask and maintain physical distance when traveling."

Closer to home, check with your local government, businesses and school districts. In some states, such as Florida, mask mandates have been banned but those bans are being challenged. Florida's ban was overturned by a Tallahassee Circuit Court judge who ruled that the governor overreached his authority, misinterpreted state law and ignored scientific evidence in issuing his order.

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More:McDonald's requires masks again for customers and employees in areas with high COVID transmission

What type of masks are best for preventing COVID?

To be an effective block against the spread of COVID, your mask should have two or more layers of washable, breathable fabric, it should completely cover your nose and mouth and fit snugly without gaps, and ideally should have a nose wire to pinch the top over your nose to prevent air from leaking out of the top.

Do not choose masks made of mesh, or out of material you can't breathe through. Masks with vents or exhalation valves will just let droplets get out.

Face gaiters aren't the best, but the CDC suggests wearing one with two layers, or doubling them over. There have not yet been reliable studies on the effectiveness of face shields.

From the CDC: Your guide to masks

Contributing: Adrianna Rodriquez

C. A. Bridges is a Digital Producer for the USA TODAY Network. Follow him on Twitter at @cabridges