• Henning Bundgaard, Johan Skov Bundgaard, Daniel Emil Tadeusz Raaschou-Pedersen, Christian von Buchwald, Tobias Todsen, Jakob Boesgaard Norsk, Mia M Pries-Heje, Christoffer Rasmus Vissing, Pernille B Nielsen, Ulrik C Winsløw, Kamille Fogh, Rasmus Hasselbalch, Jonas H Kristensen, Anna Ringgaard, Porsborg AndersenMikkelM0000-0001-6162-5205Nordsjaellands Hospital, Hillerød, and Aalborg University Hospital, Aalborg, Denmark (M.P.A., C.T.)., Nicole Bakkegård Goecke, Ramona Trebbien, Kerstin Skovgaard, Thomas Benfield, Henrik Ullum, Christian Torp-Pedersen, and Kasper Iversen.
• The Heart Center, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark (H.B., J.S.B., D.E.T., M.M.P., C.R.V., U.C.W., A.R.).
• Ann. Intern. Med. 2021 Mar 1; 174 (3): 335-343.

BackgroundObservational evidence suggests that mask wearing mitigates transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It is uncertain if this observed association arises through protection of uninfected wearers (protective effect), via reduced transmission from infected mask wearers (source control), or both.ObjectiveTo assess whether recommending surgical mask use outside the home reduces wearers' risk for SARS-CoV-2 infection in a setting where masks were uncommon and not among recommended public health measures.DesignRandomized controlled trial (DANMASK-19 [Danish Study to Assess Face Masks for the Protection Against COVID-19 Infection]). (ClinicalTrials.gov: NCT04337541).SettingDenmark, April and May 2020.ParticipantsAdults spending more than 3 hours per day outside the home without occupational mask use.InterventionEncouragement to follow social distancing measures for coronavirus disease 2019, plus either no mask recommendation or a recommendation to wear a mask when outside the home among other persons together with a supply of 50 surgical masks and instructions for proper use.MeasurementsThe primary outcome was SARS-CoV-2 infection in the mask wearer at 1 month by antibody testing, polymerase chain reaction (PCR), or hospital diagnosis. The secondary outcome was PCR positivity for other respiratory viruses.ResultsA total of 3030 participants were randomly assigned to the recommendation to wear masks, and 2994 were assigned to control; 4862 completed the study. Infection with SARS-CoV-2 occurred in 42 participants recommended masks (1.8%) and 53 control participants (2.1%). The between-group difference was -0.3 percentage point (95% CI, -1.2 to 0.4 percentage point; P = 0.38) (odds ratio, 0.82 [CI, 0.54 to 1.23]; P = 0.33). Multiple imputation accounting for loss to follow-up yielded similar results. Although the difference observed was not statistically significant, the 95% CIs are compatible with a 46% reduction to a 23% increase in infection.LimitationInconclusive results, missing data, variable adherence, patient-reported findings on home tests, no blinding, and no assessment of whether masks could decrease disease transmission from mask wearers to others.ConclusionThe recommendation to wear surgical masks to supplement other public health measures did not reduce the SARS-CoV-2 infection rate among wearers by more than 50% in a community with modest infection rates, some degree of social distancing, and uncommon general mask use. The data were compatible with lesser degrees of self-protection.Primary Funding SourceThe Salling Foundations.

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