The first case of Covid-19 in the US might not have been the infection that sparked the outbreak that followed, according to a new analysis. Instead, research suggests that these early cases in Washington quickly fizzled out and the wider outbreak in the state was ignited by cases that arrived in the US weeks later.
Scientists from the University of Arizona carried out a genetic analysis of SARS-CoV-2 types found in Europe and the US and simulated the spread of the virus through contact networks, allowing them to gain some insight into the outbreak transmission network. The report, which has not yet been peer-reviewed, can be read on the pre-print server biorXiv.
As it currently stands, the first confirmed case of a Covid-19 infection in the US was on January 20 after a Chinese national traveled to Snohomish County in Washington State from Wuhan, the city in China where the outbreak is widely thought to have originated. This new modeling does not change this, but it does suggest the early case effectively reached a dead end and did not directly lead to the larger outbreaks that ripped through the state weeks later.
This first case is known as WA1. Out of the hundreds of genomes sequences in Washington, the study authors note “no viruses with genomes identical to WA1 or transitional between it and the outbreak clade... had been sampled there.” In other words, WA1 was not genetically similar to the viruses that were spread through the Washington outbreak clade.
The second sequenced virus, known as WA2, was obtained from a sample collected five weeks later on February 24 from a teenager in Snohomish County. Importantly, WA2 appears to contain a couple of notable mutations that were also found among dozens of the later Covid-19 cases in Washington State, indicating the WA2 patient was part of a chain of transmission that later ran wild in the state.
Previous research has found that WA2 is subtly different to WA1, leading the scientists to conclude that WA1 had circulated around Washington State for several weeks, picking up a few mutations along the way, before exploding in a large number of cases weeks later. But this new paper puts forward another argument: the rate of mutations suggests that the WA2 group arrived in Washington from China sometime around February 13. The WA1, however, was relatively well contained and burned out.
In a strange twist of fate, Washington’s initial (and, according to this study, wrong) assumptions about the outbreak starting with WA1 might have actually ended up saving them from further infections.
“One irony is the beneficial impact the decision of government officials in Washington State, to be among the first in the US to initiate restrictions on social distancing and size of gatherings, had even though the decision was founded at least in part on an assumption about the timing of community transmission not supported by the phylogenetic data,” the study authors write.
“This action may have closed the gap between the onset of sustained community transmission and mitigation measures in Washington State, compared to other locales like New York City, in ways that deserve careful reevaluation.“
As ever, this is unlikely to be the end of the story since the Covid-19 timeline is constantly changing as more information and insights come to light. While it can appear disconcerting and confusing to see the accepted consensus change so rapidly, this simply shows that science is doing its job — with new evidence comes new theories and new conclusions.