More recently, studies conducted in hamsters seem to show that "higher doses of administered virus led to more severe manifestations of COVID-19," Gandhi and Rutherford wrote. And when the hamsters were protected with simulated masking, they "were less likely to get infected, and if they did get infected, they either were asymptomatic or had milder symptoms than unmasked hamsters," the experts noted.
For ethical reasons, similar trials in humans haven't been conducted. But population studies seem to support the "mask as vaccine" theory. For example, the U.S. Centers for Disease Control and Prevention reported that by mid-July about 40% of coronavirus infections were asymptomatic, but in areas of the United States where mask wearing was very prevalent, that number rose to 80%.
In early outbreaks of SARS-CoV-2 infections on cruise ships, before the widespread use of face masks, the rate of cases with no symptoms was about 20%, Gandhi and Rutherford noted. But in an outbreak on one Argentinian cruise ship where face masks were mandated for passengers and crew, the rate of asymptomatic cases rose sharply, to 81%.
Finally, in two recent outbreaks in U.S. food-processing plants where workers were told to wear masks, 95% of cases of coronavirus infections were asymptomatic, and the remaining 5% experienced only mild-to-moderate symptoms, the two experts said.
