November 19, 2020

Danish study: Effectiveness of masks in preventing COVID-19

By: Judy Mathias

Editor's Note

This Danish unblinded, randomized, controlled trial finds that wearing masks to supplement other public health measures did not reduce the COVID-19 infection rate among wearers.

Researchers randomized 3,030 participants to wear three-layer, disposable surgical masks with ear loops that were made in China, and 2,994 were assigned to be controls with no masks.

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Participants in the mask group were instructed to wear a mask when outside the home for a month. Wearing a mask was in addition to measures already in place that included quarantining those with COVID-19, social distancing, frequent hand hygiene and cleaning, and limiting visitors to hospitals and nursing homes. Cafes and restaurants were closed until May 18, during the study period of April 3 to June 2.

COVID-19 infection occurred in 42 (1.8%) masked participants and 53 (2.1%) control participants. In the mask group, 9 (0.5%) were positive for one or more of 11 respiratory viruses other than COVID-19, compared with 11 (0.6%) in the control group. Positivity for any virus, including COVID-19, occurred in 9 (0.5%) masked participants versus 16 (0.8%) control participants.

A recommendation to wear a surgical mask when outside the home did not reduce, at conventional levels of statistical significance, incident COVID-19 infection compared with no mask recommendation, the researchers say. The findings offer evidence of 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.


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