Effects of social distancing on COVID-19 incidence and mortality rates

From the Newcomb and VoPham Group, Public Health Sciences Division

Because we are currently in the midst of a pandemic, most states have implemented policies to increase social distancing in order to reduce transmission of severe acute respiratory syndrome coronavirus 2, SARS-CoV-2.  Social distancing includes prohibiting crowds of people at non-essential businesses, limiting social gatherings to 10 people or less, ordering citizens to stay at home, and continuously maintaining a 6-foot distance from others.  Although these actions may reduce the incidence of disease, there have been personal and economic consequences due to the implementation of these policies. Currently, statistics on the mortality and morbidity of SARS-Cov-2 are updated regularly.  It’s important to keep the public informed on the progression of the virus. However, social distancing interventions and its effects on COVID-19 morbidity and mortality in the US have not received intense research focus. Understanding the effectiveness of these interventions is critical to decisions on whether to relax or reinstate these policies. Therefore, the first author, Dr. VoPham and others conducted a study to examine the associations between statewide policies and objective measures of social distancing. Also, the group examined the association between objective social distancing and COVID-19 incidence and mortality in the US. The article is presently available on the medRxiv preprint server; the authors are actively working to update and publish this work.

In order to objectively measure county-level social distancing, de-identified smartphone GPS data were provided by Unacast, which incorporated the following: change in distance traveled (per device), change in non-essential venue visitation (e.g. clothing store, nail salons), and the proximity of two users less than 50 meters apart and within a 60-minute time interval.  To adequately calculate the change in movement over a period of time, the measures were compared to the same day of the week before COVID-19; these were the four weeks prior to March 8, 2020.  Distancing was scored 1-5; the higher the number, the higher the distance relative to the same day of the week before COVID-19. The exclusion criteria consisted of 1) counties with a population less than 1,000 (data were not provided for these counties), 2) counties where less than 100 smartphones were observed for 70% of the days before COVID-19, or 3) counties with less than 5 non-essential venues or less than 100 non-essential venue visits before COVID-19. The Johns Hopkins Coronavirus Resource Center provided COVID-19 incidence and mortality data per county. The timeframe of this analysis was from February 24, 2020 to April 29, 2020 (on this date, a few states re-opened their businesses).  

Graphical representation of County-level objective social distancing prior to the first confirmed COVID-19 case, changes in 110 objective social distancing before and after stay-at-home guidance, and COVID-19 incidence in the US.
County-level objective social distancing prior to the first confirmed COVID-19 case, changes in 110 objective social distancing before and after stay-at-home guidance, and COVID-19 incidence in the US. Image from Vopham

Social distancing data were available for 3,054 counties (94%) in all 50 states and Washington, D.C as shown in Figure 1.  A total of 45 states, including Washington D.C., implemented stay-at-home orders. Following implementation of stay-at-home orders, there was a 35% increase in social distancing (p <0.001).  Each one unit increase in social distancing was associated with a decrease in COVID-19 incidence (adjusted incidence rate ratio, IRR: 0.71, 95% CI 0.57-0.87) and mortality (IRR: 0.65, 95% CI 0.55-0.76). 

The results from this study were consistent with other studies using a different social distancing methodology.  This study adds to the previous evidence by using a nationally representative dataset that objectively measured social distancing via GPS. In Iowa, there wasn’t an implementation of a stay-at-home order; therefore, the incidence of COVID-19 increased rapidly. This example demonstrates the potential benefits of implementing social distancing policies in the United States. Policies across 45 states and Washington, D.C. significantly improved the outcome for most American lives.  This study also demonstrated the effectiveness of public health measures and informs the implications of opening each state. 

This research was supported by supported in part by the National Institutes of Health and the Brigham Research Institute Fund to Sustain Research Excellence.

Fred Hutch/UW Cancer Consortium members Trang VoPham, Emily White, and Polly Newcomb contributed to this study.

VoPham T, Weaver MD, Hart JE, Ton M, White E, Newcomb PA. 2020. Effect of social distancing on COVID-19 incidence and mortality in the US. MedRxiv. doi:10.1101/2020.06.10.20127589