Reluctance to endorse mask wearing to slow transmission of SARS-Cov-2 has been rationalized by the failure of randomized control trials (RCTs) to provide supportive evidence . In contrast, a mechanism-based approach suggests that mask wearing should be expected to reduce transmission: so that contrary evidence from RCTs likely reflects the need to focus policy attention on addressing interacting or mediating factors that offset the basic positive effect . The differing conclusions that result from these two approaches reflect the limitations of RCT-based approaches that are compounded in scenarios, such as pandemics, where urgent decisions are required with limited evidence.