Back in the early days of the COVID-19 pandemic, researchers in Denmark decided to conduct an usual experiment to test the infection-fighting power of face masks the same way they'd evaluate a potential vaccine or drug.
At the time, mask use was not recommended by Danish health authorities, and fewer than 5% of residents used them outside of hospital settings. Those conditions made it possible to conduct the first — and only — randomized controlled trial of the face coverings.
The researchers recruited more than 6,000 volunteers from around the country who spent at least three hours each day with people from other households and didn't wear masks for their jobs. About half of these volunteers were chosen at random to receive 50 surgical face masks and were asked to wear them whenever they left home for the next month. The other half did not get masks and served as controls.
Overall, 95 of the 4,862 volunteers who made it to the end of the study became infected with SARS-CoV-2, the coronavirus that causes COVID-19. That's an infection rate of just under 2%.
But no matter how the researchers sliced and diced their data, they could not find a strong signal that the volunteers in the mask group were more protected than their counterparts in the control group.
In a typical clinical trial, this is the point where researchers would say their intervention didn't work. But in this case, the investigators went the other way.
The problem, they said, wasn't with the masks. The problem was that people didn't use masks enough.
The study results "should not be used to conclude that a recommendation for everyone to wear masks in the community would not be effective in reducing SARS-CoV-2 infections," wrote the team led by researchers from Copenhagen University Hospital.
A trio of current and former editors of Annals of Internal Medicine, the journal that published the study, went further.
"Masks likely need to be worn by most if not all people to reduce community infection rates," they wrote. "The results of this trial should motivate widespread mask wearing to protect our communities and thereby ourselves."