Most public health authorities around the world now urge people to wear face masks to reduce transmission of COVID-19. Some hesitated to give this recommendation at the beginning of the pandemic. Frequently, health care professionals could not get enough high-quality masks. As a result, policy-makers tried to make sure the public would not snap up limited supplies. In addition, some experts pointed out a scarcity of scientific studies on the effectiveness of face masks.
Denmark Trial Tests Ability of Face Masks to Reduce Transmission:
Now Danish researchers have published the results of a randomized controlled trial they completed last spring (Annals of Internal Medicine, Nov. 18, 2020). They recruited 6,000 volunteers in April, 2020. Then the investigators randomly assigned the participants either to wear a mask or not when outside the home. They instructed both groups to wash their hands and practice social distancing.
The investigators supplied a pack of 50 high-quality surgical masks to each person who was supposed to wear them. Each participant also received instructions on how to wear them effectively. The volunteers also tested themselves for SARS-CoV-2 virus and for antibodies at the beginning and end of the study. Nearly 5,000 people completed all the requirements for this study.
At the time of this trial, most Danes did not wear masks in public. As a result, some mask-wearing participants reported that they got comments or funny looks. (However, the decision to wear a mask or not was not politicized as it was in the US.) Nonetheless, a majority of volunteers (93 percent) wore their masks as recommended or mostly as recommended outside their homes.
The Surprising Results of the DANMASK-19 Trial:
The scientists found that 1.8 percent of the masked volunteers (42 out of 3030) and 2.1 percent of those who did not wear masks (53 of 2994) contracted COVID-19 during the month of the study. This was similar to the community rate of infection at that time. The difference is not statistically significant.
What This Study Means and What It Does Not Tell Us:
The DANMASK-19 study was designed for a situation when few people in the community wear face coverings. The researchers wanted to know if face masks reduce transmission of COVID-19 in that context from 2 percent to 1 percent. The results are inconclusive, so we can’t say for sure that face masks DON’T protect people. The study was also relatively short. One month might not be enough time to assess effectiveness.
The investigators emphasize:
“These findings do offer evidence about the degree of protection mask wearers can anticipate in a setting where others are not wearing masks and where other public health measures, including social distancing, are in effect. The findings, however, 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, because the trial did not test the role of masks in source control of SARS-CoV-2 infection.”
In other words, we still don’t know whether face masks reduce transmission of COVID-19 if everyone wears them. The study did not test that situation. This study does suggest that just wearing a mask will not provide strong protection without other preventive tactics.
Consequently, we think that it makes sense to visit outside rather than inside, at a distance rather than close. If people must be inside with others, they might do better if all of them wear surgical or N95 masks. (Here’s a list.) We also believe it makes sense to prioritize hand washing and ventilation.
People who feel ill should stay away from others, but even people with no symptoms can spread SARS-CoV-2. As a result, each of us should take precautions as if we were infected, because we might be without knowing it. If we all exercise extra caution, more people may survive until COVID-19 vaccines become widely available to help lower the likelihood of infection.