Half were given surgical masks and told to wear them when leaving their homes; the others were told not to wear masks in public.
At that time, 2 percent of the Danish population was infected — a rate lower than that 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 masks would cut the infection rate by half among wearers. Instead, 42 people in the mask group, or 1.8 percent, got infected, compared with 53 in the unmasked group, or 2.1 percent. 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 a cardiologist at the University of Copenhagen. “Not a lot.”
Dr. Mette Kalager, a researcher at Telemark Hospital in Norway and the Harvard School of Public Health, was persuaded. The study showed that “although there might be a symbolic effect,” she wrote in an email, “the effect of wearing a mask does not substantially reduce 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 the effectiveness of masks for wearers may have been harder to detect. Participants reported their own test results; mask use was not independently verified, and users may not have worn them correctly.
“There is absolutely no doubt that masks work as 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 C.D.C., who wrote an editorial outlining weaknesses of the research.