Half received surgical masks and were told to wear them when leaving their homes; others were told not to wear masks in public.
At that time, 2 percent of the Danish population was infected – a lower rate than in many places in the United States and Europe today. Social distancing and frequent hand washing were common, but masks were not.
About 4,860 participants completed the study. The researchers had hoped that the masks would halve the infection rate in carriers. Instead, 42 people in the mask group, or 1.8%, were infected, compared to 53 in the unmasked group, or 2.1%. The difference was not statistically significant.
“Our study gives an indication of how much you gain from wearing a mask,” said Dr. Henning Bundgaard, lead author of the study and cardiologist at the University of Copenhagen. “Not a lot.”
Dr Mette Kalager, a researcher at Telemark Hospital in Norway and Harvard School of Public Health, was convinced. The study showed that “while there may be a symbolic effect,” she wrote in an email, “the effect of wearing a mask does not significantly reduce the risk” for wearers.
Critics were quick to note the study’s limitations. Among them: The incidence of infections in Denmark was lower than it is today in many places, meaning that the effectiveness of masks for wearers may have been more difficult to detect. Participants reported their own test results; mask use has not been independently verified and users may not have worn them correctly.
“There is no doubt that masks work like source control,” preventing people from infecting others, said Dr Thomas Frieden, chief executive of Resolve to Save Lives, an advocacy group, and former director of the CDC, who wrote an op-ed describing weaknesses in the research.