Novel mobility index tracks COVID-19 transmission following stay-at-home orders

Abstract

Considering the emergence of SARS-CoV-2 variants and low vaccine access and uptake, minimizing human interactions remains an effective strategy to mitigate the spread of SARS-CoV-2. Using a functional principal component analysis, we created a multidimensional mobility index (MI) using six metrics compiled by SafeGraph from all counties in Illinois, Ohio, Michigan and Indiana between January 1 to December 8, 2020. Changes in mobility were defined as a time-updated 7-day rolling average. Associations between our MI and COVID-19 cases were estimated using a quasi-Poisson hierarchical generalized additive model adjusted for population density and the COVID-19 Community Vulnerability Index. Individual mobility metrics varied significantly by counties and by calendar time. More than 50% of the variability in the data was explained by the first principal component by each state, indicating good dimension reduction. While an individual metric of mobility was not associated with surges of COVID-19, our MI was independently associated with COVID-19 cases in all four states given varying time-lags. Following the expiration of stay-at-home orders, a single metric of mobility was not sensitive enough to capture the complexity of human interactions. Monitoring mobility can be an important public health tool, however, it should be modelled as a multidimensional construct.

Figure 1

The average daily changes from baseline in the six mobility metrics for all counties of each state between January and December 2020. The baseline was calculated using a rolling average of the 7 previous days. The solid vertical lines represent the date the stay-at-home orders were put in place while the dotted vertical lines represent the dates the stay-at-home orders were lifted.

Figure 2

MI values for each county of each state on the day the stay-at-home orders expired (reopen) and on July 4, 2020. Blue shades indicate a decrease in mobility (MI < 0) and red shades indicate an increase in mobility (MI > 0).

Figure 3

Model results comparing the MI and its association with COVID-19 cases and a commonly used single metric of mobility (fraction of devices leaving home). For each state, the left panel summarizes the multidimensional MI; the right panel represents the percentage of devices leaving their home (x-axis); y-axis is the adjusted incidence rate ratio of COVID-19, at varying lagged response (0–21 days) (z-axis).