Estimating the effect of physical distancing on the COVID-19 pandemic using an urban mobility index

Soucy, Jean-Paul R.; Sturrock, Shelby L.; Berry, Isha; Daneman, Nick; MacFadden, Derek R.; Brown, Kevin A.

Governments around the world are implementing population-wide physical distancing measures in an effort to control transmission of COVID-19, but metrics to evaluate their effectiveness are not readily available. We used a publicly available mobility index based on the relative frequency of trips planned in a popular transit application to evaluate the effect of physical distancing on infection growth rates and reproductive number in 34 states and countries. We found that a 10% decrease in relative mobility in the 2nd week of March was associated with a 11.8% relative decrease (exp(β) = 0.882; 95% CI: 0.822, 0.946) in the average daily growth rate in the fourth week of March and a change in the instantaneous reproductive number of -0.054 (95% CI: 0.097, -0.011) in the same period. Our analysis demonstrates that decreases in urban mobility were predictive of declines in epidemic growth at national or sub-national scales. Mobility metrics offer an appealing method to calibrate population-level physical distancing policy and implementation.