Path of Destruction: How the coronavirus traveled from coast to coast, bringing the US to a standstill
Domestic spread was a significant source of COVID-19 infections in the US, say researchers who found strong evidence that outbreaks on the East Coast (Connecticut) are linked to outbreaks on the West Coast (Washington).
The research team says the findings indicate that "interconnectedness (between states) will perpetuate the domestic spread of SARS-CoV-2" and that domestic spread will likely become the primary source of new infections in the US. The analysis comes at a time when the US has released a plan to open the country in phases.
To uncover the sources of SARS-CoV-2 introductions and patterns of spread within the US, the research team sequenced nine viral genomes from early reported COVID-19 patients in Connecticut.
Experts reason that due to its proximity to several high-volume airports, southern Connecticut is a suitable location to test this hypothesis. The analysis shows that the COVID-19 outbreak in Connecticut was driven, in part, by domestic virus introduction.
The analysis also places the majority of these genomes with viruses sequenced from Washington state. "We sequenced nine viral genomes from early reported COVID-19 patients in Connecticut."
"Our phylogenetic analysis places the majority of these genomes with viruses sequenced from Washington state. By coupling our genomic data with domestic and international travel patterns, we show that early SARS-CoV-2 transmission in Connecticut was likely driven by domestic introductions."
"Moreover, the risk of domestic importation to Connecticut exceeded that of international importation by mid-March regardless of our estimated impacts of federal travel restrictions," say researchers from the Yale School of Public, US, University of Basel, Switzerland, Yale University, US, University of Birmingham, UK, and University of Washington, US, among other institutes.
"This study provides evidence for widespread, sustained transmission of SARS-CoV-2 within the US and highlights the critical need for local surveillance," the team says in their findings published in the Cell.
The novel coronavirus SARS-CoV-2, which causes COVID-19, was first detected in the Pacific Northwest region of the US in January 2020, with subsequent COVID-19 outbreaks detected in all 50 states by early March.
Researchers say that according to the testing criteria adopted by the US Centers for Disease Control and Prevention (CDC), the majority of the early COVID-19 cases in the US were either associated with travel to a "high-risk" country or close contacts of previously identified cases.
"In response to the risk of more travel-associated cases, the US placed travel restrictions on multiple countries with SARS-CoV-2 transmission, including China on January 31, Iran on February 29, and Europe on March 11."
"However, community transmission of SARS-CoV-2 was detected in the US in late February when a California resident contracted the virus despite meeting neither testing criteria," says the study.
Experts further say that from March 1 to 19, 2020, the number of reported COVID-19 cases in the US rapidly increased from 74 to 13,677, and the virus was detected in all 50 US states.
It was estimated that the true number of COVID-19 cases in the US is likely in the tens of thousands, suggesting substantial undetected infections and spread within the country.
Accordingly, the researchers hypothesized that with the growing number of COVID-19 cases in the US and the large volume of domestic travel, new US outbreaks "are now more likely to result from interstate rather than international spread".
"By sequencing SARS-CoV-2 from local cases and comparing their relatedness to virus genome sequences from other locations, we used ‘genomic epidemiology’ to identify the likely sources of SARS-CoV-2 in Connecticut. We supplemented our viral genomic analysis with airline travel data from major airports in southern New England to estimate the risk of domestic and international importation therein."
"Our data suggest that the risk of domestic importation of SARS-CoV-2 into this region far outweighs that of international introductions regardless of federal travel restrictions, and find evidence for coast-to-coast US SARS-CoV-2 spread," says the team.
Experts argue that, though simplistic, their model demonstrates the urgent need to focus control efforts in the US on preventing further domestic virus spread.
As the epidemic progresses, domestic introductions of the virus could undermine control efforts in areas that have successfully mitigated local transmission, the researchers warn.