Masks, mechanisms and Covid-19: the limitations of randomized trials in pandemic policymaking.

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.