Differential Effects of Intervention Timing on COVID-19 Spread in the United States

Assessing the effects of early non-pharmaceutical interventions(1–5) on COVID-19 spread in the United States is crucial for understanding and planning future control measures to combat the ongoing pandemic(6–10). Here we use county-level observations of reported infections and deaths(11), in conjunction with human mobility data(12) and a metapopulation transmission model(13,14), to quantify changes of disease transmission rates in US counties from March 15, 2020 to May 3, 2020. We find significant reductions of the basic reproductive numbers in major metropolitan areas in association with social distancing and other control measures. Counterfactual simulations indicate that, had these same control measures been implemented just 1–2 weeks earlier, a substantial number of cases and deaths could have been averted. Specifically, nationwide, 56.5% [95% Cl: 48.1%−65.9%] of reported infections and 54.0% [95% Cl: 43.6%−63.8%] of reported deaths as of May 3, 2020 could have been avoided if the same control measures had been implemented just one week earlier. We also examine the effects of delays in re-implementing social distancing following a relaxation of control measures. A longer response time results in a stronger rebound of infections and death. Our findings underscore the importance of early intervention and aggressive response in controlling the COVID-19 pandemic.