Reconciling the efficacy and effectiveness of masking on epidemic outcomes

Mask wearing in public settings has been broadly implemented as a means to mitigate the COVID-19 pandemic. However, the reported effectiveness of masking has been much lower than laboratory measures of efficacy, and this large discrepancy has cast doubt on the utility of masking. Here, we develop an agent-based model that comprehensively accounts for individual masking behaviors and infectious disease dynamics, and test the impact of masking on epidemic outcomes. Using realistic inputs of mask efficacy and contact data at the individual level, the model reproduces the lower effectiveness as reported in randomized controlled trials. Model results demonstrate that transmission within households, where masks are rarely used, can substantially lower effectiveness, and reveal the interaction of nonlinear epidemic dynamics, control measures (e.g., masking and social distancing), and potential measurement biases. Overall, model results show that, at the individual level, consistent masking can reduce the risk of first infection, and, over time, reduce the frequency of repeated infection. At the population level, masking can provide direct protection to mask wearers, as well as indirect protection to non-wearers, collectively reducing epidemic intensity. These findings suggest it is prudent for individuals to use masks during an epidemic, and for policy makers to recognize the less-than-ideal effectiveness of masking when devising public health interventions. Significance statementDuring the COVID-19 pandemic, mask wearing in public settings has been a key control measure. However, the low effectiveness reported for masking has cast doubt on its validity. Here, we develop an agent-based model to interrogate influencing factors. Using realistic inputs of mask efficacy and contact data, the model reproduces the lower effectiveness reported in real-world settings. Testing shows that transmission within-household where masks are rarely used can substantially lower effectiveness. Nonetheless, the model results support the effectiveness of masking at both the individual and population levels, albeit at less-than-ideal levels. Overall, these findings indicate it is prudent for individuals to use masks during an epidemic, and for policy makers to recognize the less-than-ideal effectiveness of masking when devising interventions.