Researchers in Denmark reported on Wednesday that surgical masks did not protect the wearers against infection with the coronavirus in a large randomized clinical trial. But the findings conflict with those from a number of other studies, experts said, and is not likely to alter public health recommendations in the United States.
The study, published in the Annals of Internal Medicine, did not contradict growing evidence that masks can prevent transmission of the virus from wearer to others. But the conclusion is at odds with the view that masks also protect the wearers — a position endorsed just last week by the Centers for Disease Control and Prevention.
Critics were quick to note the study’s limitations, among them that the design depended heavily on participants reporting their own test results and behavior, at a time when both mask-wearing and infection were rare in Denmark.
From early April to early June, researchers at the University of Copenhagen recruited 6,024 participants who had been tested beforehand to be sure they were not infected with the coronavirus.
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.
Characteristics of Participants Completing the Study
Source: Henning Bundgaard, DMSc, et al.The participants in the face mask group were all given high-quality surgical masks with a filtration rate of 98%. Worth noting is that this quality of mask is even better than the N95 masks that some use today, and it is many times better than the cloth masks that we are told to wear “for our own safety.”
The members of the control group did not wear masks.
What were the results?
- 1.8% of those who wore masks got infected with COVID-19.
- 2.1% of those who did not wear masks got infected with COVID-19.
Dr. Christine Laine, editor in chief of the Annals of Internal Medicine, described the previous evidence that masks protect wearers as weak. “These studies cannot differentiate between source control and personal protection of the mask wearer,” she said.
Dr. Laine said the new study underscored the need for adherence to other precautions, like social distancing. Masks “are not a magic bullet,” she said. “There are people who say, ‘I’m fine, I’m wearing a mask.’ They need to realize they are not invulnerable to infection.” Here is the conclusion of the Danish study in the authors’ words:
"Our results suggest that the recommendation to wear a surgical mask when outside the home among others did not reduce, at conventional levels of statistical significance, the incidence of SARS-CoV-2 infection in mask wearers in a setting where social distancing and other public health measures were in effect…"