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Universal community masking has become an iconic and highly politicized means of preventing SARS-CoV-2 transmission. Nonetheless, the science of masking has yet to catch up with the policy of masking: Most evidence of community effectiveness comes from observational data, simulations, modeling (e.g., Mask Math), or extrapolations from healthcare settings. Further, the relative contributions of source control (preventing spread from infected people to others) and protection for uninfected wearers is not well understood.
To explore the latter, Danish researchers constructed a randomized trial of community masking during spring 2020, a time of relatively low disease prevalence in Denmark, when quarantining and social distancing were recommended…