We can exploit randomized controlled trials, compartmental models, and spillovers The only approaches currently available to reduce transmission of the novel coronavirus severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) are behavioral: handwashing, cough and sneeze etiquette, and, above all, social distancing. Policy-makers have a variety of tools to enable these “nonpharmaceutical interventions” (NPIs), ranging from simple encouragement and recommendations to full-on regulation and sanctions. However, these interventions are often used without rigorous empirical evidence: They make sense in theory, and mathematical models can be used to predict their likely impact ( 1 , 2 ), but with different policies being tried in different places—often in complicated combinations and without systematic, built-in evaluation—we cannot confidently attribute any given reduction in transmission to a specific policy.