Researchers in Denmark said on Wednesday that surgical masks did not protect users against coronavirus infection in a large randomized clinical trial. But the findings contradict those of a number of other studies, experts say, and are unlikely to change public health recommendations in the United States.
The study, published in the Annals of Internal Medicine, does not contradict the growing evidence that masks can prevent the transmission of the virus from the carrier to others. But the conclusion contradicts the view that masks also protect consumers, a position approved last week by the Centers for Disease Control and Prevention.
Critics were quick to point out the study’s limitations, including that the design depended heavily on participants reporting their own test results and behavior at a time when wearing masks and infection were rare in Denmark.
Coronavirus infections are on the rise in the United States, and even officials who have opposed the mandates are reversing. Approximately 40 states have introduced some mask requirements, according to a database maintained by The New York Times.
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, advocates for a national mandate, as does President-elect Joseph R. Biden Jr.
“I will not be president until January 20, but my message to everyone today is this: wear a mask,” Mr Biden recently wrote on Twitter.
From early April to early June, researchers at the University of Copenhagen recruited 6,024 participants who were pre-tested to make sure they were not infected with the coronavirus.
Half were given surgical masks and told to wear them when they left their homes; the others were told not to wear masks in public.
At that time, 2% of the Danish population was infected – a percentage lower 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 hoped the masks would halve the rate of infection among consumers. Instead, 42 people in the mask group, or 1.8%, became infected, compared with 53 in the non-mask group, or 2.1%. The difference is not statistically significant.
“Our study shows how much you earn from wearing a mask,” said Dr. Henning Bundgaard, lead author of the study and a cardiologist at the University of Copenhagen. “Not many.”
Dr. Mete Kalager, a professor of medical decision-making at the University of Oslo, found the study convincing. The study showed that “although it may have a symbolic effect,” she wrote in an email, “the effect of wearing a mask does not significantly reduce the risk” for wearers.
Other experts are not convinced. The incidence of infections in Denmark was lower than it is today in many places, which means that the effectiveness of masks for carriers may have been more difficult to detect, they note.
Participants reported their own test results; the use of the mask has not been independently verified and users may not have worn them properly.
“There is absolutely no doubt that masks act as a 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 editorial. weaknesses of the study.
The question on which this study was developed is: Do they work as a personal protection? The answer depends on what mask is used and what kind of exposure to the virus each person has, said Dr. Frieden, and the study is not intended to extract these details.
“The N95 mask is better than the surgical mask,” said Dr. Frieden. “The surgical mask is better than most fabric masks. A fabric mask is better than nothing. “
The conclusion of the study flies in the face of other studies that suggest that masks really protect the user. In its recent bulletin, the CDC cited a dozen studies that found that even fabric masks could help protect the consumer. Most of them were laboratory tests of particles blocked by materials of different types.
Susan Ellenberg, a biostatist at the University of Pennsylvania, Perelman Medical School, noted that the protection provided by consumer masks tends to be “beneficial” in the study, even if the results are not statistically significant.
“Nothing in this study tells me it’s useless to wear a mask,” she said.
Dr. Elizabeth Haloran, a statistician at the Fred Hutchinson Cancer Research Center in Seattle, said the usefulness of the masks also depends on how much virus a person is exposed to.
“If you show this article to a health care provider who works in a Covid ward in a hospital, I doubt she or he would say that this article convinces them not to wear a mask,” she said.
But Dr. Christine Line, editor-in-chief of the Annals of Internal Medicine, described previous evidence that masks protect consumers as weak. “These studies cannot distinguish between control of the source and personal protection of the mask holder,” she said.
Dr Laine said the new study highlights the need to comply with other precautions, such as 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 that they are not invulnerable to infection. “