July 1 (UPI) -- Americans staying home -- even before local officials imposed social distancing restrictions -- likely stemmed the spread of COVID-19 in many parts of the country, according to an analysis published Wednesday by the Lancet Infectious Diseases.
In the 25 counties most affected by the outbreak by mid-April, movement by individuals dropped by up to 63 percent following the first three months of the outbreak, based on mobile phone data, according to the researchers.
These counties began to see declines in cases of COVID-19, the disease caused by SARS-CoV-2, nine to 12 days after the declines in movement began. This, researchers said, reflects the five- to 14-day period between infection and symptoms appearing.
"Our results strongly support the conclusion that social distancing played a crucial role in the reduction of case growth rates in multiple U.S. counties during March and April, and is therefore an effective mitigation policy for COVID-19," study co-author Lauren M. Gardner said in a press release.
"Critically, we also found that behavioral changes were already underway in many U.S. counties days to weeks before state-level or local-level stay-at-home policies were implemented," said Gardner, an associate professor of engineering at Johns Hopkins University.
Within the first four months of COVID-19 being reported in the United States, the virus spread to every state and more than 90 percent of counties, according to Gardner and her colleagues.
In general, policy response was highly "decentralized" during this period, researchers said. This means instructions on county and state-level stay-at-home policies were implemented in varying ways and levels, making the effectiveness of social distancing difficult to assess, they said.
For the study, researchers used real-world mobile phone movement data as an indicator of social distancing. This was used to compare local case growth with how individuals actually modified their movement patterns, rather than "relying on assumed compliance with local stay-at-home policies," they said.
Daily mobility data from Jan. 1 through April 20 was taken from mobile network records to capture trends in movement patterns for each U.S. county, and compared to baseline patterns pre-COVID-19 to generate a social distancing baseline, the researchers said.
Individuals began reducing their movement in all 25 of the most-affected counties six to 29 days before state-level stay-at-home policies were implemented, the researchers said.
Between January 24 and April 17, compared to normal levels, individual mobility dropped by a range of 35 percent in New York City -- the lowest figure in the study -- to 63 percent in Harris County, Texas, which includes Houston -- the highest figure in the study.
In general, individuals modifying their behavior helped slow the spread of the virus more quickly than if they had waited until the implementation of stay-at-home policies, the researchers said.
"If individual-level actions were not taken and social distancing behavior 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," Gardner said. "This demonstrates that it is within the power of each U.S. resident to help slow the spread of COVID-19."