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

Authors: Sen Pei, Sasikiran Kandula, Jeffrey Shaman*
Department of Environmental Health Sciences, Mailman School of Public Health,
Columbia University

Assessing the effects of early non-pharmaceutical interventions1-5 on COVID-19 spread
in the United States is crucial for understanding and planning future control measures to
combat the ongoing pandemic6-10. Here we use county-level observations of reported infections and deaths11, in conjunction with human mobility data12 and a metapopulation transmission model13,14, 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, 61.6%
[95% CI: 54.6%-67.7%] of reported infections and 55.0% [95% CI: 46.1%-62.2%] 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.->....

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