Wearing masks does not prevent COVID-19 infection in areas with public health measures in effect

Wearing surgical masks in addition to other public health measures is not associated with a significant reduction in SARS-CoV-2 infection rate among wearers in a community with modest infection rates, some degree of social distancing, and uncommon general mask use, according to a Danish study.

“Our results suggest that the recommendation to wear a surgical mask when outside the home among others did not reduce, at conventional levels of statistical significance, the incidence of SARS-CoV-2 infection in mask wearers in a setting where social distancing and other public health measures were in effect, mask recommendations were not among those measures, and community use of masks was uncommon,” the researchers said.

Adults spending more than 3 hours per day outside the home without occupational mask use between April and May 2020 were included in this randomized controlled trial. Of the participants, 3,030 were randomly assigned to the recommendation to wear masks and 2,994 to control; 4,862 completed the study. In addition, all participants were encouraged to follow social distancing measures for COVID-19.

Forty participants recommended to wear masks (1.8 percent) and 53 controls (2.1 percent) were infected with SARS-CoV-2, with a between-group difference of –0.3 percentage points (95 percent confidence interval [CI], –1.2 to 0.4; p=0.38) and an odds ratio of 0.82 (95 percent CI, 0.54–1.23; p=0.33). Similar results were seen in multiple imputation accounting for loss to follow-up. [Ann Intern Med 2021;doi:10.7326/M20-6817]

While the difference observed was not statistically significant, the 95 percent CIs were compatible with a 46-percent reduction to a 23-percent increase in infection.

Of note, the intervention lasted for only 1 month and was conducted during a period when Danish authorities recommended the quarantine of diagnosed patients, physical distancing, and hand hygiene as general protective means against SARS-CoV-2 transmission. [www.sst.dk/da/Udgivelser/2020/COVID-19-Forebyggelse-af-smittespredning]

“Yet, the findings were inconclusive and cannot definitively exclude a 46-percent reduction to a 23-percent increase in infection of mask wearers in such a setting,” the researchers said. “It is important to emphasize that this trial did not address the effects of masks as source control or as protection in settings where social distancing and other public health measures are not in effect.”

The face masks used by the participants were high-quality surgical masks with a filtration rate of 98 percent. In a recent meta-analysis, no statistically significant difference was noted between respirators (N95 or FFP2) and surgical face masks in preventing influenza among healthcare workers. [J Int Soc Respir Prot 2009;26:54-70; J Evid Based Med 2020;13:93-101]

On the other hand, several studies have shown that mask wearing conferred some protection from the transmission of SARS-CoV-2. Such measure is most effective at reducing the spread of the virus when compliance is high. [Proc Natl Acad Sci U S A 2021;118:e2014564118; Travel Med Infect Dis 2020;36:101751; mSphere 2020;5:e00637-20]

The present study was limited by inconclusive 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.