Masks Do Not Protect Wearer From COVID-19

Face masks provide limited protection to the wearer against COVID-19, according to new data. However, these findings do not mean masks should not be worn, the researchers noted.

The randomized control trial took place between April 3 to June 2, 2020, in Denmark. During this period, public health measures included quarantining if a person was infected with SARS-CoV2, social distancing, reducing the amount of people seen, frequent hand hygiene, and limiting visits to hospitals and nursing homes. Mask wearing, however, was not a commonly practiced measure during the study.

Study participants were recruited via media advertisements, private companies, and public organizations. Participants completed a baseline demographic survey and were then randomly assigned 1:1 to the mask or control group. Persons instructed to wear masks were provided with 50 three-layer, disposable surgical face masks. All participants were provided with the materials needed to conduct self-collected antibody testing and oropharyngeal/nasal swab samples. Participants self-reported the results of the at-home antibody test, both at baseline and at 1 month.

The primary outcome of the study was SARS-CoV2 infection, defined as a positive result from an oropharyngeal/nasal swab test, a positive SARS-CoV2 antibody test, or a hospital diagnosis of SARS-CoV2.

A total of 6024 persons were eligible for the study, which was split into 2 sections based off of month. The first group (n = 2995) were followed from mid-April to mid-May and the second group (n = 3029) were followed from the beginning of May until the beginning of June. Combined over both time periods, 3030 participants were instructed to wear masks, and 2994 were instructed not to wear face masks.

Adherence was self-reported with 46% wearing masks as recommended, 47% wearing them predominantly as recommended, and 7% reporting not wearing them as recommended.

The primary outcome occurred in 1.8% of participants wearing masks and 2.1% in the control group. SAR-CoV2 diagnosis occurred in 22 participants who reported wearing their mask as instructed and in 40 participants who reported nonadherence to masks.

Despite these findings, this does not indicate that masks are ineffective. This study was designed to analyze whether mask usage protected individuals from becoming infected with COVID-19. The study did not analyze source control (eg, if wearing a mask when infected keeps others around the sick individual safe.) As mentioned, public health orders at the time of the study did not mandate mask usage. This means that participants were largely exposed to persons not wearing masks.

Ultimately, these findings are inconclusive. The power of the primary outcome was 0.38 (a power of <0.05 was designated as statistically significant) and the confidence interval ranged from -1.2 to 0.4. The confidence interval is compatible with a 46% decrease to a 23% increase in infection. Again, it is important to note that the study did not address the use of masks as source control or analyze the use of masks in a setting where social distancing or other public health measures are not in effect.

—Audrey Amos, PharmD

Reference:

Bundgaard H, Bundgaard JS, Raaschou-Pedersen DET, vonBuchwald C, et al. Effectiveness of adding a mask recommendation to other public health measures to prevent SARS-CoV2 infection in danish mask wearers: a randomized controlled trial. Ann Int Med. Published online November 18, 2020. https://doi.org/10.7326/M20-6817