Social Distancing Appears to Be Making an Impact in 4 Major Cities

New data from the US Centers for Disease Control and Prevention (CDC) sheds light on the impact of coronavirus disease 2019 (COVID-19) mitigation efforts in 4 major metropolitan areas of the United States.

The data, contained in the CDC’s Morbidity and Mortality Weekly Report, provide an early look into efforts to stop the still-ongoing pandemic. The report focuses on New York City, New Orleans, San Francisco, and Seattle. It finds the average 3-day percentage change of cases generally declined as social distancing measures were put in place and as more people stayed home in each community.

Corresponding author, Kathleen A. Ethier, PhD, a member of the CDC’s COVID-19 response team, and colleagues write that they hope that looking at the connections between policies and behavior—as well as behavior and case outcomes—helps public health and governmental officials gain a better understanding of what works when trying to contain outbreaks.

“Monitoring adherence to community mitigation strategies through mobility measures could improve the understanding of the types, combinations, and timing of policies that are associated with slowing the spread of COVID-19 as well as other infectious diseases,” they say.

The report was compiled using a variety of sources, including the USA Facts case data aggregator, Google news alerts for policy changes, and publicly available mobility information based on aggregated, anonymized mobile phone movement data. The study spanned the period from February 26 through April 1.

The 4 cities tended to institute similar policies within days of each other. For instance, King County (where Seattle is located) closed its schools on March 12^th, Louisiana closed schools on the 13^th, and New York closed its schools on March 16^th. Though California did not issue a mandatory statewide school closure, an executive order that prompted many schools to close was issued March 13^th.

New York State, Washington State, and Louisiana all put in place statewide limits on bars and restaurants on March 16^th. California followed on March 19^th.

Ethier and colleagues noted that mid-March was a period of significant tightening of restrictions in many states, as governors and other officials began to understand the immediacy of the potential threat.

“This awareness and concern also likely impacted the public, potentially leading to further decreases in mobility,” they write.

Washington State was the first of the 4 states to issue a statewide disaster declaration, and it also saw the earliest drop in its 3-day average percentage change of cases, peaking in early March. New York had the latest drop in its 3-day average, with a peak of March 19^th.

In all 4 cities, roughly 80% of people were leaving home on a daily basis at the start of the study period, in late February. By April 1^st, the rate in all of the cities had dipped by 20 to 30 percentage points in all cities.

The report noted a number of limitations. Chief among them, reported case counts don’t include people who might have the disease and be undiagnosed. However, the authors said these data do lend early credence to efforts undertaken so far to limit the scale of the crisis.

“Better understanding of the short- and long-term impact of the community disruption that results from these measures is critical,” they wrote.

“However, this analysis suggests that policies to increase social distancing when case counts are increasing can be an important tool for communities as changes in behavior result in decreased spread of COVID-19.”