Fig. 7

Population movement is influenced by COVID incidence and is highly heterogeneous across states. The large left-hand column shows summaries of cases, movement, and county opening and closing in New Jersey and serves as a guide to interpreting the other panels. New Jersey saw a surge in incidence in early April (cases per 100,000, 7-day running mean, grey shade, top panel). We distinguish between the focal county, which is the county receiving visitors from listed visiting counties, which are recorded in the SafeGraph data set. We can then calculate the proportion of visiting counties which have more cases than the focal county being visited for churches, gyms, grocery stores, parks, and bars (colored lines in the top left panel). The frequency of trips from higher-incidence to lower-incidence counties appear to lead daily incidence. Finally, we look at the magnitude of population flux in response to incidence in the bottom panel. Grey lines are the 95th, 75th, 50th, 25th, and 5th quantiles of the difference in the numbers of cases between the focal county and the visiting county. There is substantial variation in the numbers of population movement across states. The green line indicates the average number of unique visiting counties per focal county for that state. While most states saw a rebound in the number of unique visiting counties, Vermont maintained a low number of influx of population