Effectiveness of Adding Mask Recommendation to Prevent SARS-CoV-2

Quick Takes

  • This randomized trial on recommending surgical masks for prevention of SARS-CoV-2 infection was performed in the setting of low community transmission.
  • Adherence to masking was very poor (<50%) and likely overestimated in this study given reporting biases.
  • These data should not be used to conclude that the use of masks is ineffective at reducing risk of SARS-CoV-2 infection.

Study Questions:

Does the use of surgical masks in settings where mask use is uncommon reduce the risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection?


DANMASK-19 (Danish Study to Assess Face Masks for the Protection Against COVID-19 Infection) was an investigator-initiated, nationwide, unblinded, randomized controlled trial. Participants (adults ≥18 years who spent ≥3 hours a day outside home) were randomized 1:1 to instruction to wear a surgical face mask (TYPE II EN 14683 [Abena]; filtration rate, 98%; made in China) when outside the home, or to following the national guidelines in Denmark from April 3 to June 2, 2020, which did not recommend the use of masks in the community. The mask group received 50 masks. Both groups received materials and instructions for SARS-CoV-2 polymerase chain reaction (PCR) testing and antibody testing on receipt and at 1 month. The primary outcome was SARS-CoV-2 infection, defined as a positive PCR, antibody test, or hospital diagnosis.


A total of 3,030 participants were randomly assigned to the recommendation to wear face masks, and 2,994 were assigned not to wear face masks, with 4,862 participants (80.7%) completing the study. In the mask group, 46% of participants wore the mask as recommended. Forty-two (1.8%) participants in the mask group and 53 (2.1%) in the control group met the primary outcome of SARS-CoV-2 infection (odds ratio, 0.82; 95% confidence interval, 0.54-1.23; p = 0.33). In post hoc analyses including only those who wore the mask as recommended, infection occurred in 22 (2.0%) participants in the mask group and 53 (2.1%) in the control group.


The recommendation to wear surgical masks as a supplement to other public health measures in a setting of low infectivity rate did not reduce SARS-CoV-2 infection.


While the authors should be commended for designing and attempting to implement a nationwide clinical trial on the use of masks in COVID-19, there is unfortunately little that we can derive from their findings. The study was conducted during a time of low SARS-CoV-2 community spread, with an average daily infection rate in Denmark of <100 for May and June, and consequently a low event rate for the study. Adherence to masking, reported as <50%, is likely an overestimate, given the social response bias. Another important aspect not addressed by this study is the potential of masking to mitigate the severity of COVID-19 through decreasing the viral load at exposure. It is thus impossible to use these data to make the claim that masks are ineffective at reducing risk of infection, especially in situations of high community spread such as that occurring today throughout the United States.

Keywords: Coronavirus Infections, COVID-19, Filtration, Masks, Polymerase Chain Reaction, Primary Prevention, Public Health, Risk Reduction Behavior, Severe Acute Respiratory Syndrome, severe acute respiratory syndrome coronavirus 2, Viral Load

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