Social distancing measures to address the U.S. COVID-19 epidemic may have significant health, social, and economic impacts. This study estimates the mean change in state-level COVID-19 epidemic growth before versus after the implementation of statewide social distancing measures in the United States.

Using interrupted time-series analysis, the authors examine the impact of time in relation to implementation of the first statewide social distancing measure on COVID-19 growth rate, which was calculated as the log of daily COVID-19 cases minus the log of daily COVID-19 cases on the prior day. The pre-implementation period began 14 days prior to implementation and included up to 3 days after implementation to account for incubation. Post-implementation began 4 days after, up to and including March 30.

The found that all states applied some form of statewide social distancing between March 10-27. The mean daily COVID-19 growth rate decreased beginning four days after implementation of the first statewide social distancing measures, by an additional 0.8% per day; 95% CI, -1.4% to -0.2%; P=0.002).

This reduction corresponds to an increase in doubling time of the epidemic from 3.3 days (before) to 5.0 days (at 14 days after implementation).

They conclude that statewide social distancing measures were associated with a decrease in U.S. COVID-19 epidemic growth. Based on the size of the epidemic at the time of implementation in each state, social distancing measures were associated with a decrease of 3,090 cases at 7 days, and 68,255 cases at 14 days, after implementation.