A genomic analysis of SARS-CoV-2 in San Diego and Mexico reveals that physical distancing was more effective than international border closures in containing the virus.
Scripps Research scientists and colleagues sequenced more than 82,000 SARS-CoV-2 samples gathered from routine genomic surveillance in San Diego and the state of Baja California, Mexico, to reconstruct viral spread dynamics from March 2020 to the end of the first Omicron surge in December 2022.
The team then compared the genomes to those from elsewhere in the world and integrated air and land travel data from anonymous cellphone tracking. The US/Mexico border was closed to nonessential traffic from March 2020 to November 2021.
The results were published yesterday in Cell.
Mandate relaxation led to more distant travel
During stay-at-home and physical distancing mandates early in the pandemic, SARS-CoV-2 spread primarily within and between adjacent counties. But as mandates eased, people started to travel farther, and COVID-19 spread from distant locations rose.
"We found that the relative import risk from neighboring locations was low at the beginning of the pandemic, peaked during the spring of 2020, and decreased steadily from then until the end of available data (74.8% in April 2020 to 36.9% in July 2021)," the researchers wrote.
SARS-CoV-2 spread continuously between San Diego and Mexico throughout the pandemic, which the researchers said suggests that the partial US/Mexico border closure was ineffective.
We found that the relative import risk from neighboring locations was low at the beginning of the pandemic, peaked during the spring of 2020, and decreased steadily from then until the end of available data.
"After the mandates were relaxed, about 50% of the virus circulating in San Diego was the result of locally circulating virus, and 50% had been recently introduced as the result of travel between locations," first author Nathaniel Matteson, PhD, of Scripps Research, said in a news release.
The study authors said the results underscore the importance of collaborative interventions for pandemic prevention and containment. "We show that it's not necessarily how geographically close locations are to each other; the real measure is how connected two locations are in terms of the movement of people," co-senior author Mark Zeller, PhD, of Scripps Research, said in the release.
