Whether masks work is a different question from whether mask mandates work. The effectiveness of mandating an even partially effective intervention depends on many factors, and the impact of the intervention can be challenging to demonstrate. If adherence to a public health mandate is low, a mandate is unlikely to have an impact (seat belts reduce the risk of death, if they are worn). Higher rates of indoor masking in parts of Asia (eg, Hong Kong, Japan, Korea, and Singapore) may account for lower rates of infection and death, especially early in the pandemic. For a population in which use of the intervention is already common, a statistically significant reduction in infection rates will be more difficult to establish. Furthermore, assessment of the effectiveness of mask mandates requires either cluster randomized studies or ecologic studies in which the unit of observation is the group, not the individual. Such studies have been done: rigorous evaluations of mask mandates in several settings suggested substantial protective benefits. In Germany, an opportunity to generate high-quality data arose when different regions mandated masking at different times during the COVID-19 pandemic. Mask mandates were associated with a 45% reduction in SARS-CoV-2 infections. Variation in timing of mask mandates across the United States provided a similar study opportunity, and a matched cohort analysis of more than 400 US counties showed that enactment of a mask mandate was associated with a 25% reduction in COVID-19 incidence 4 weeks later. Although it is possible that cases might soon peak without intervention if masking is implemented when incidence is increasing, US communities with mask mandates had less transmission than those without mandates after controlling for potential confounders, including premandate incidence.
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The risk-benefit calculations that shape public health recommendations may differ by setting and may change over time (Table 2). When the COVID-19 pandemic began, scarcity of medical masks and respirators precluded their use outside of health care settings. There was concern that community members wearing masks might self-contaminate with SARS-CoV-2 or might fail to practice other public health measures due to a false sense of security. However, although respiratory viruses can contaminate the outside of masks when masks are worn for hours in high-exposure clinical settings, the relevance of this finding to community settings is unclear. There is no compelling evidence that masking is associated with neglect of other public health measures; in fact, studies have suggested the opposite.
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Other concerns raised about masks included possible impacts on respiratory function; although masking can be uncomfortable, especially in warm conditions, there is no compelling evidence of consequential deleterious effects on physiology, including during exercise. It can be difficult for young children to wear well-fitting masks, and the possibility that masking may impede cognitive and social development suggests that this risk should be considered, balancing with possible benefits of masking. There is abundant evidence that school closures are deleterious to children’s health and that masking in schools decreases transmission of SARS-CoV-2 within schools.
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Although available evidence strongly suggests that masking in the community can reduce the spread of SARS-CoV-2, knowledge gaps persist. It is challenging to disentangle the impacts of masks from those of other interventions on transmission of SARS-CoV-2. The effectiveness of masks may differ between variants of SARS-CoV-2. Until recently, respirators such as N95s were not widely available outside health care settings. We lack precise estimates of the extent to which the community spread of SARS-CoV-2 is reduced at different levels of uptake of different mask types in different contexts. However, there is alignment between findings from laboratory models and limited available effectiveness data: a study on the use of masks or respirators in indoor public settings17 showed that respirators were more protective against SARS-CoV-2 infection than surgical masks, which were more protective than cloth masks.
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