Masks mean people get a lower dose of the virus. And evidence is growing that this ‘lower dose’ means they should get ill less severely, says an expert at a major US university.

Masks protect other people from your own germs, which is especially important to keep unknowingly infected people from spreading the coronavirus.

But now, there’s mounting evidence that masks also protect you.

If you’re unlucky enough to encounter an infectious person, wearing any kind of face covering will reduce the amount of virus that your body will take in.

Breathing in a small amount of virus may give you a milder infection. But inhaling a huge volume of virus particles can result in serious disease or death.

That’s the argument Dr. Monica Gandhi, University of California San Francisco professor of medicine, is making. She explains why — if you do become infected with the virus — masking can still protect you from more severe disease.

Coronavirus can be widely spread by people who are not visibly sick — either because they haven’t yet shown signs of illness, or they will spend the entire course of their infections with little or no symptoms at all.

A key piece of evidence for this emerged earlier this year, on the Diamond Princess cruise ship that carried infected crew and passengers in Asia. A study published by the U.S. Centers for Disease Control and Prevention found that of 712 people testing positive for the virus, nearly half were asymptomatic at the time of testing.

“We also know that viral load is highest early during disease,” said Dr. Chaz Langelier, an assistant professor at UC San Francisco, during the panel discussion. In fact, 44% of transmissions are believed to occur when the infected person has no symptoms, according to a study published in Nature Medicine.

That’s different from the seasonal flu, where peak infectiousness occurs about one day after the onset of symptoms, Langelier said.

Masks don’t filter out all viral particles, Gandhi said. But even cloth face masks filter out a majority of viral particles.

And even if a person wearing a mask gets infected, the mask — by filtering out most of the viral particles exhaled by the infected person — probably leads to less severe disease, Gandhi said.

The idea that a lower dose of virus when being infected brings less illness is a well-entrenched idea in medicine.

Even going back to 1938, there was a study showing that by giving mice a higher dose of a deadly virus, the mice were more likely to get severe disease and die, Gandhi said.

The same principle applies to humans. A study published in 2015 gave healthy volunteers varying doses of a flu virus; those who got higher doses got sicker, with more coughing and shortness of breath, Gandhi said.

And another study suggested that the reason the second wave of the 1918-19 flu pandemic was the deadliest in the U.S. was because of the overcrowded conditions faced in Army camps as World War I wound down.

“In 1918, the Army camps [were] characterized by a high number of contacts between people and by a high case-fatality rate, sometimes 5 to 8 times higher than the case-fatality rate among civilian communities,” the study said.

Finally, a study published in May found that surgical mask partitions significantly reduced the transmission of the coronavirus among hamsters. And even if the hamsters protected by the mask partitions acquired the coronavirus, “they were more likely to get very mild disease,” Gandhi said.

It may mean that even if there’s a rise in coronavirus infections in a city, the masks may limit the dose of virus people are getting and result in less severe symptoms of illness.

That’s what Gandhi said she suspects is happening in San Francisco, where mask wearing is relatively robust. Further observations are needed, Gandhi said.

There’s more evidence that masks can be protective — even when wearers do become infected. She pointed to an outbreak at a seafood plant in Oregon where employees were given masks, and 95% of those who were infected were asymptomatic.

Gandhi also cited the experience of a cruise ship that was traveling from Argentina to Antartica in March when the coronavirus infected people on board, as documented in a recent study. Passengers got surgical masks; the crew got N95 masks.

But instead of about 40% of those infected being asymptomatic — which is what would normally be expected — 81% of those testing positive were asymptomatic, and the masking may have helped reduce the severity of disease in people on board, Gandhi said.

The protective effects are also seen in countries where masks are universally accepted for years, such as Taiwan, Thailand, South Korea and Singapore. “They have all seen cases as they opened … but not deaths,” Gandhi said.

The Czech Republic moved early to require masks, issuing an order in mid-March, Gandhi said. In the Czech Republic, “every time their cases would go up …their death rate was totally flat. So they didn’t get the severe illness with these cases going on.”

By May, the Czech Republic lifted its face mask rule. “And they’re doing great,” Gandhi said.

The experts were first quoted in the LA Times.

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