It's not as if there's no reason to believe face masks can prevent the spread of COVID-19. Scientists say the coronavirus moves from person to person mainly through the air, either on respiratory droplets or smaller, aerosolized particles. If either of these makes their way into the respiratory tract, an infection can take off.
People with active infections are most contagious in the first few days after exposure, often before they develop symptoms. That's why the World Health Organization, the U.S. Centers for Disease Control and Prevention and other public health authorities say it's important to wear a mask whenever you're in close contact with people from other households, even if you don't feel sick.
Widespread mask use is credited with keeping the COVID-19 death toll in the double digits in places such as New Zealand (25 deaths to date), Singapore (28 deaths) and Vietnam (35 deaths). In Taiwan, where officials ramped up mask production just a few weeks after the WHO announced the existence of the novel coronavirus, only seven people have died of COVID-19.
"Use of face masks has emerged as a powerful tool to reduce the health and economic harms of the pandemic," Dr. Thomas Frieden, a former CDC director who now leads the nonprofit health initiative Resolve to Save Lives, and his colleague Dr. Shama Cash-Goldwasser wrote in a commentary that accompanied the study.
If masks work so great, why did they make so little difference in Denmark? The study authors and their allies offer multiple explanations.
The trial was conducted in two waves, with the first group testing masks between April 14 and May 15 and the second group testing them between May 2 and June 2. During most of that time — until May 18 — restaurants and cafes in the country were closed. Stores were open and public transportation was operational, but customers were advised to maintain social distancing. In addition, there were limits on social gatherings as well as visits to hospitals and nursing homes.
With all these measures in place, the added benefit of wearing masks might be negligible when the community prevalence of the virus is low, the researchers wrote. (At the time of the study, the daily incidence of new infections was four times lower in Denmark than it was in the U.S.)
Masks are believed to help in two ways. Although they can protect wearers from the incoming germs, their primary benefit is their ability to prevent the wearer's germs from spreading to others.
Since few Danish residents were wearing masks during the study period, volunteers were mostly exposed to maskless people who could spread a virus easily — and that was true regardless of whether they were randomly assigned to wear a mask or not.
Another problem is that people assigned to wear masks often chose not to. Only 46% of volunteers in the mask group told the researchers they followed all the rules about wearing masks in public, 47% said they "predominantly" wore their masks, and 7% said they didn't follow the rules.