Getting to the Truth About the Effectiveness of Masks in Preventing COVID-19

Preventing COVID-19 Before the COVID-19 pandemic, medical masks were just devices worn with the aim of preventing respiratory infection. During the pandemic, masking recommendations differed widely and changed rapidly across settings, and masks became a controversial political symbol. Laboratory studies that examined the physical properties of masks abounded but were generally small, often involved robotic mask wearers, and examined the spread of particles rather than the spread of infection. Some (but not all) observational studies comparing communities with andwithout prevalent masking suggested an association of masking with less COVID-19, but it is challenging for such studies to disentangle whether findings were due to masks, to unmeasured confounding, or to concomitant physical distancing and other measures. There are only 3 published trials examining the effectiveness of various masking interventions to reduce SARS-CoV-2 infection in real-world settings—2 in community settings (1, 2) and 1 in hospitals (3). Although trials provide the highest quality of evidence, the evolving pandemic with changes in natural immunity, vaccination availability, and different virus variants presented substantial challenges to arriving at definitive answers to the question about the effectiveness of masks in preventing SARS-CoV-2 infection (4). In their article, Chou and Dana report the final update of a living systematic review that attempts to answer this question (5). They identified the 3 randomized trials cited earlier and 21 observational studies that controlled for confounders (7 in community and 4 in patient care settings) that examined risk for SARS-CoV-2 infection. They conclude that mask interventions may be associated with a small reduction in risk for SARS-CoV-2 infection in community settings and that risk for infection in patient care settings seems similar with surgical masks and N95 respirators. Substantial caveats accompany both conclusions. Chou and Dana’s review differed from a recently published Cochrane review (6) in that it focused on studies examining masking interventions and SARS-CoV-2, whereas the Cochrane review examined studies of various physical interventions, including masks, to reduce viral respiratory illness in general. Chou and Dana examined evidence published through 2 January 2023 and provided qualitative results, whereas the Cochrane group examined evidence through October 2022 and provided quantitative estimates. The Cochrane review reported with moderate certainty that wearing masks in the community probably makes little or no difference in the transmission of influenza-like or COVID-19–like illness (risk ratio, 0.95 [95% CI, 0.84 to 1.09]) and were very uncertain that there was any difference between N95 respirators and medical or surgical masks in preventing the spread of viral respiratory illness. Both reviews identified a lack of evidence examining potential harms associated with masking interventions. Both reviews are careful, systematic examinations of the current state of evidence for the effectiveness of masking interventions in preventing COVID-19 that suggest a substantial amount of uncertainty and identify important evidence gaps. Yet, some people state with certainty that masking is effective, and others state with equal certainty that it is not. How do we get at the truth? First, we must clarify that people are talking about the same thing. Are people referring to the effectiveness of a mask to prevent passage of virus or to prevent SARSCoV-2 infection? Are they referring to the effectiveness of masks themselves or of masking recommendations or policies? Certainty that masks are effective may rely on studies of viral penetration, whereas certainty of mask ineffectiveness may rely on studies of masking recommendations. Second, we must consider masking as 1 element of multicomponent strategies to reduce transmission of infection and how accompanying strategies may affect mask effectiveness. Third, we need to be explicit in weighing benefits and harms. Although some may consider any evidence of benefit sufficient to justify masking, others believe that inconvenience, discomfort, or communication impedance outweigh small benefits. Policymakers making masking recommendations and establishing policy should use structured evidence to decision frameworks that clearly acknowledge and consider uncertainty and balance benefits and harms (7). As we emerge from COVID-19 being a public health emergency, we must acknowledge the inevitability of future viral epidemics and to reflect on the types of studies that could better inform us about the effectiveness of masking interventions. Randomized trials of masking interventions are challenging and demand pragmatism, but existing trials show that they are feasible. To know whether masks are effective, we need to examine them in real-world situations. A mask may block virus in a laboratory, but that doesn’t tell us how masks function when worn by real people in real-world settings. A mask that is more permeable to virus may function better than one that is impermeable to virus if the more permeable mask is easier to wear properly and consistently. Masks are recommended to prevent transmission of infection, so studies should examine clinically meaningful infection outcomes. Interpretation of study results demands a priori definition of clinically meaningful differences so that even when CIs cross 1, the degree to which the observed outcomes are compatible with meaningful differences can be assessed. Mask recommendations occur in the context of other infection control measures, infection prevalence, and vaccination rates. Studies of masking, whether observational or interventional, must recognize and account for these concomitant measures and contextual factors (8, 9). Alternative study designs, such as process evaluation trials or realist trials may inform what conditions are necessary for masking