Do Face Masks Reduce Coronavirus Spread?

August 10, 2020

For the first time, the U.S. Centers for Disease Control and Prevention (CDC) has recommended that even seemingly healthy people wear masks over their mouths and noses when they step out of their homes to places where it is difficult to keep their distance from others. But there is still significant controversy over the extent to which masks – particularly the homemade fabric masks that the CDC recommends to the public – can slow the spread of SARS-CoV-2 (the virus that causes COVID-19).

Researchers write in two new papers that attempt to address the efficacy of the masks, one more rigorous than the other, and come to different conclusions. One study looked at the effect of masks on seasonal coronavirus (which causes many cases of the common cold) and found that surgical masks helped reduce the amount of virus spread by patients. Another specifically looked at SARS-CoV-2 and found that neither surgical masks nor fabric masks had any effect on reducing the spread of the virus, but only with four participants and using a crude measure of viral transmission.

The bottom line, experts say, is that masks may help keep people with COVID-19 from unknowingly passing the virus. But evidence for the efficacy of surgical or homemade masks is limited, and masks aren’t the most important protection against coronavirus.

“Wearing a mask doesn’t mean you stop other practices,” says May Chu, a clinical professor of epidemiology at the Colorado School of Public Health in Anschutz Medical Campus, who was not involved in any of the new studies.” It doesn’t mean you’re closer to people, it doesn’t mean you don’t have to wash your hands as often, you can touch your face. All of that is still there, it’s just an added feature.”

The basics of face masks
Recommendations on masks can be confusing because all masks are not the same.N95 masks are effective in preventing the spread of viruses. This mask, when worn correctly, fits snugly against the face and filters out 95% of particles 0.3 microns or larger. But N95 masks are in serious shortage even among medical professionals, who are most exposed to SARS-CoV-2 and most in need of the strongest protection against the virus. They are also difficult to wear properly. For these reasons, the CDC does not recommend them for general use.

The CDC also does not recommend the use of surgical masks for the general public due to shortages. These masks do not provide a seal to the face, but include a non-woven polypropylene layer that provides a moisture barrier. In surgical masks, about 70 percent of the outside air passes through the mask and about 30 percent bypasses the sides of the mask, Chu told Live Science.For this reason, they don’t offer the same protection as N95.

That leaves the fabric masks, which the CDC currently recommends for general use. Fabric masks also allow air around the sides, but lack the non-woven, moisture-repelling layer. They block only about 2 percent of the airflow coming in, Zhu says.

All of these surgeries and leaks in fabric masks are why public health officials generally don’t believe wearing a mask can prevent anyone from contracting a virus already floating in the environment. The airflow follows the path of least resistance, said Rachael Jones, an associate professor of family and preventive medicine at the University of Utah, who was not involved in the new study. If virus particles are nearby, they have a simple path around the surgery or a fabric mask. In the case of a fabric mask, the wearer is likely to drift into particles small enough to flow through the fabric.

But what about the reverse? When a mask wearer coughs or sneezes, the barrier may be large enough to hold a large amount of the nasty substance initially sprayed – even if there are gaps in the fabric or on the sides. That’s the problem the new mask study aims to solve. Both surgical masks and fabric masks work well to suppress viruses.

Effectiveness of masks

The study also returned the curious result that most swabs from the outside of the patient’s mask tested positive for coronavirus, while most swabs from the inside tested negative. Jones said the authors speculated that perhaps the jet of air turbulence from coughing carried the virus to the outside of the mask, but that explanation is not very satisfying.

Another study, published in the April 3 issue of the journal Nature Medicine, used a more sophisticated method to collect virus particles emitted by patients. Researchers asked 426 volunteers to breathe into a cone-like device for 30 minutes to capture all exhaled gas. Of these, 43 patients had influenza, 54 had rhinovirus, and 17 had seasonal coronavirus (the kind that causes the flu, not the kind that causes COVID-19). This method allowed the researchers to quantify how much virus was found in droplet particles, which were larger than 0.0002 inches (5 microns) in diameter, versus aerosol particles, which were 5 microns or smaller. Participants were randomly assigned to either wear a surgical mask or not wear a surgical mask.

“Wearing a mask doesn’t mean you stop other practices,” says May Chu, a clinical professor of epidemiology at the Colorado School of Public Health in Anschutz Medical Campus, who was not involved in any of the new studies.” It doesn’t mean you’re closer to people, it doesn’t mean you don’t have to wash your hands as often, you can touch your face. All of that is still there, it’s just an added feature.”

The basics of face masks
Recommendations on masks can be confusing because all masks are not the same.N95 masks are effective in preventing the spread of viruses. This mask, when worn correctly, fits snugly against the face and filters out 95% of particles 0.3 microns or larger. But N95 masks are in serious shortage even among medical professionals, who are most exposed to SARS-CoV-2 and most in need of the strongest protection against the virus. They are also difficult to wear properly. For these reasons, the CDC does not recommend them for general use.

A study published April 6 in the journal Annals of Internal Medicine found that they are not. The study, led by Korean researchers, involved asking four patients with COVID-19 to cough into a Petri dish 7.8 inches (20 cm) away. Patients coughed without a mask while wearing a disposable surgical mask and then a 100% cotton mask.

Neither mask meaningfully reduced the viral load from coughing onto the Petri dish. However, experts contacted by Live Science who were not involved in the study were hesitant to comment on the results. Zhu said the researchers didn’t look beyond 7.8 inches to see if the droplets didn’t travel as far when people wore the masks.

“They didn’t measure 2 feet or 3 feet or 4 feet,” she said.

The study also returned the curious result that most swabs from the outside of the patient’s mask tested positive for coronavirus, while most swabs from the inside tested negative. Jones said the authors speculated that perhaps the jet of air turbulence from coughing carried the virus to the outside of the mask, but that explanation is not very satisfying.

Another study, published in the April 3 issue of the journal Nature Medicine, used a more sophisticated method to collect virus particles emitted by patients. Researchers asked 426 volunteers to breathe into a cone-like device for 30 minutes to capture all exhaled gas. Of these, 43 patients had influenza, 54 had rhinovirus, and 17 had seasonal coronavirus (the kind that causes the flu, not the kind that causes COVID-19). This method allowed the researchers to quantify how much virus was found in droplet particles, which were larger than 0.0002 inches (5 microns) in diameter, versus aerosol particles, which were 5 microns or smaller. During the study, participants were randomly assigned to wear surgical masks or no masks at all.

The first key finding was that the researchers were able to detect viruses in the tiny aerosol particles in all cases: influenza, rhinovirus, and coronavirus. In the case of influenza, they cultured the captured particles and found them to be infectious. Ben Coughlin, the study’s author and head of the Department of Epidemiology and Biostatistics at the University of Hong Kong, says this is important because health professionals have long debated whether influenza can be transmitted by aerosols. The study suggests that it probably can, and that influenza may be able to.

“For seasonal coronaviruses and rhinoviruses, we didn’t try to culture the virus in aerosols, but there’s no reason to believe the virus wouldn’t be contagious,” Cowling told Live Science.

And as for the masks? For influenza, surgical masks reduced the amount of virus the patient released in droplets, but not in aerosols; masks reduced coronavirus in droplets and aerosols; and for rhinovirus, masks did not reduce any of the viruses. For seasonal coronaviruses, the researchers found viruses in droplets from three out of 10 samples of participants who did not wear masks, and in four out of 10 aerosols from samples that did not wear masks. Viruses were not detected in either droplets or aerosols in the samples with masks.

The differences between the viruses may be related to where these infectious invaders make their home in the respiratory tract, said Coughlin, who is also co-director of the WHO Collaborating Centre for Infectious Disease Epidemiology and Control. For example, a virus that multiplies deep in the lungs may need to travel in smaller particles to make it all the way to the world, while a virus that multiplies primarily in the nose and throat may be more easily mobilized in larger droplets.

Results from influenza and seasonal coronavirus suggest that surgical masks can help keep COVID-19 from spreading the virus in people, Coughlin said.SARS-CoV-2 may behave similarly to the virus he and his team studied, he said, and the argument that people can spread the virus before they experience symptoms is an argument for recommending masks for everyone.

Still, experts have mixed feelings about the potential usefulness of non-N95 masks.

“To me, there’s no harm in wearing these masks, but in terms of this study, there’s no benefit,” Jones said. She said the sample size for seasonal coronavirus was small, and there was a large non-mask-related variation in the amount of virus people emitted, especially given that most samples without masks had no detectable coronavirus.

One thing that everyone agrees on is that homemade fabric masks are the most ineffective, regardless of the containment provided by the non-masked person. Everyone is advised to wear a mask because “any kind of deterrent is better than none,” says Chu. But she said fabric masks are not expected to be as protective as surgical masks. That’s why public health officials are warning people to keep at least 6 feet away from masks, even if they are wearing them. In other words, homemade masks are likely to be just a small piece of the puzzle in controlling the COVID-19 pandemic.

“Enough research has been done to be able to confidently say that masks don’t stop the spread of infection, they only have a small impact on transmission,” said Coughlin.” We shouldn’t have to rely on masks to help us get back to normal.”