The Effect of Stay-at-Home Orders on COVID-19 Infections in the United States

James H. Fowler; Seth J. Hill; Nick Obradovich; Remy Levin

County-level data on cumulative COVID-19 confirmed cases were also aggregated daily by the New York Times . 17 We discarded all observations where cases were not assigned to a specific county (these account for 0.9% of total cases). We retained observations where cumulative cases declined from one day to the next due to official revisions to the counts (0.5% of cases). Figure 1b shows that the number of cases grew exponentially in each county over time from March 1, 2020 to April 16, 2020 . It also suggests that efforts to "flatten the curve" initiated in mid-to-late March might have helped to reduce the rate of exponential growth. Below we describe how we use data on cumulative cases, which include both currently and previously infectious individuals, to estimate the growth rate in the total number of currently-active infections. Availability of tests for COVID-19 in the United States has not been uniform over the date range of the study. 18 To mitigate the effect of changes in rates of testing on our measure of confirmed cases, we also collected data on the number of tests administered each day. This information is not currently available for each county, but it is available for each state by date from the COVID Tracking Project 19 for 83.7% of our observations. We merged this data with information about stay-at-home orders and confirmed cases.