Many Americans decided to start physical distancing even before statewide stay-at-home orders were issued, likely helping slow transmission of COVID-19 in the 25 hardest-hit counties, a modeling study published yesterday in The Lancet Infectious Diseases has found.
Using mobile phone data from Jan 1 to Apr 20, researchers at Johns Hopkins University identified a decline in the number of daily trips people made 6 to 29 days before states mandated that residents stay home to slow the spread of the novel coronavirus. After establishing a baseline of normal movement from Jan 8 to 31, they looked for changes through Apr 17 and assessed epidemiologic data on COVID-19 cases for each county.
Regional variation in extent of physical distancing
The observed lag of 9 to 12 days before the effects of physical distancing were seen was expected after considering the typical time from symptom onset to reporting, the authors said.
People began making far fewer daily trips in early March, well before Mar 21, when California became the first state to issue stay-at-home orders. For instance, New York City residents restricted their movements to 35% of the baseline period, the most substantial decline in movement of the 25 counties. In contrast, Harris County, Texas, which includes Houston, had the least change, to only 63% of baseline.
The regional differences may be due to the wide variation in public policy response and implementation of stay-at-home orders among counties and states during the first 4 months of the US epidemic, when the coronavirus spread to every state and more than 90% of counties, the authors said.
The 25 hardest-hit counties were located in California, Connecticut, Florida, Illinois, Louisiana, Massachusetts, Michigan, New Jersey, New York, Pennsylvania, and Texas, and largely in the county seats of large metropolitan areas.
Timeliness of state directives
While the researchers noted that the study didn't account for the benefits of other public health measures such as wearing face masks and handwashing, they said that their analysis supports physical distancing as an effective way of curbing the transmission of COVID-19.
"Our results strongly support the conclusion that social distancing played a crucial role in the reduction of case growth rates in multiple US counties during March and April, and is therefore an effective mitigation policy for COVID-19 in the USA," Lauren Gardner, PhD, MSE, said in a Lancet press release. Gardner is also the creator of the Johns Hopkins coronavirus tracker.
"If individual-level actions were not taken and social distancing behaviour was delayed until the state-level directives were implemented, COVID-19 would have been able to circulate unmitigated for additional weeks in some locations, inevitably resulting in more infections and deaths," she added.
In the press release, the authors called for "more timely, consistent and decisive policy implementation of social distancing and other known effective mitigation measures."
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Study: Many in US distanced before states mandated it - CIDRAP
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