  Social distancing remains an important strategy to combat the COVID-19
pandemic in the United States. However, the impacts of specific state-level
policies on mobility and subsequent COVID-19 case trajectories have not been
completely quantified. Using anonymized and aggregated mobility data from
opted-in Google users, we found that state-level emergency declarations
resulted in a 9.9% reduction in time spent away from places of residence.
Implementation of one or more social distancing policies resulted in an
additional 24.5% reduction in mobility the following week, and subsequent
shelter-in-place mandates yielded an additional 29.0% reduction. Decreases in
mobility were associated with substantial reductions in case growth 2 to 4
weeks later. For example, a 10% reduction in mobility was associated with a
17.5% reduction in case growth 2 weeks later. Given the continued reliance on
social distancing policies to limit the spread of COVID-19, these results may
be helpful to public health officials trying to balance infection control with
the economic and social consequences of these policies.
