Coronavirus was probably spreading widely across the US in February, new modeling data suggests, and it only took a few imported cases from other countries to set off rapid spread inside the borders.
While direct imports from China and other countries may have been responsible for the early introduction of Covid-19 to the US, most spread was state to state, researchers led by a team at Northeastern University in Boston reported.
“We estimate widespread community transmission of SARS-CoV-2 in February, 2020,” a team led by Northeastern’s Laboratory for the Modeling of Biological and Socio-technical Systems wrote on MedRxiv, a pre-print server. The work has not been peer-reviewed.
“Our results indicate that many states were seeded from domestic sources rather than international,” the team wrote. “For most of the continental states, the largest contribution of imported infections arrived through domestic travel flows.”
The US announced restrictions on travel from China on January 31. The researchers say their modeling study suggests the restrictions came far too late.
“Importations from mainland China may be relevant in seeding the epidemic in January, but then play a small role in the COVID-19 expansion in the US because of the travel restrictions imposed to/from mainland China after January 23, 2020,” they wrote.
“Domestic sources account for 85% of the virus introductions in Nebraska, 86% in New Mexico, 86% in Arkansas, and 95% in North Dakota."
The model contrasts with evidence from several studies that suggest there was already a significant number of infections in the US by the time the travel restrictions were announced on January 31, the researchers wrote.