First US Covid-19 case did not cause outbreak, disease spread later after Americans returned from China: Study

The first confirmed case In Washington state in January is not linked to the outbreak. Instead, evidence shows virus that spread in Washington reached mid-February after Americans returned to the US


                            First US Covid-19 case did not cause outbreak, disease spread later after Americans returned from China: Study
(Getty Images)

The first confirmed coronavirus infection in the US, which was discovered in January, did not trigger the outbreak that followed, suggesting that the country missed opportunities to stop Covid-19 from taking root, according to researchers in a new study. Instead, analysis suggests that the person who led to the first chain of sustained transmission in the US probably returned to the country in mid-February, nearly a month later.

"Our finding that the virus associated with the first known transmission network in the US did not enter the country until mid-February is sobering since it demonstrates that the window of opportunity to block sustained transmission of the virus stretched all the way until that point. It is clear that early interventions can have outsized effects on the course of an outbreak, and the precise impact of the slow rollout of diagnostic tests in the US on the early stages of the pandemic is likely to be explored and debated for years to come, including the initially narrow criteria for who could be tested," says the research team in their findings in a pre-print that is yet to be peer-reviewed.

"Our findings critically inform such inquiries by delineating when community transmission was first established in the US and by providing clarity on the duration of the time window before SARS-CoV-2 establishment when contact tracing and isolation might have been most effective," say researchers from the University of Arizona, Tucson; University of California San Diego, California, National Institutes of Health (NIH), Bethesda, and the University of California Los Angeles, Los Angeles. 

As of May 29, over 1,721,700 coronavirus cases have been reported in the US and more than 101,610 Americans have died in the Covid-19 pandemic.

The virus associated with the first known transmission network in the US did not enter the country until mid-February, say experts (Getty Images)

The first patient to be diagnosed with Covid-19 in the US, designated as WA1 (short for Washington state patient 1), was a person who traveled from Wuhan to Sea-Tac International Airport near Seattle, Washington state, arriving on January 15. On February 29, a SARS-CoV-2 genome was reported from a second Washington state patient, WA2, whose virus had been sampled on February 24 as part of a community surveillance study of respiratory viruses.

Previous research calculated a high probability that WA2 was a direct descendent of WA1. The analysis suggested that the new coronavirus has been circulating silently in Washington since about mid-January when the first US patient — a man who returned from Wuhan — was diagnosed. The analysis had traced the outbreak's origin to WA1, even though officials had quickly detected WA1's infection after his arrival from China on January 15, isolated him, and had done extensive contact tracing to stop transmission. 

However, modeling by researchers now suggests that WA1 did not trigger a wider outbreak. Instead, the team found evidence that the virus that spread in Washington reached the state from China in mid-February. The authors estimate that the infection that started the Seattle area outbreak arrived in the country around February 13, shortly after President Trump's ban on travel from China went into effect on February 2. The results suggest that the form of the virus that spread in Washington arrived with Americans permitted to return to the US.

"The individual who introduced the founding virus likely arrived in the US after the initiation of the 'suspension of entry' of non-US residents from China on February 2, but during the period when an estimated 40,000 US residents were repatriated from China, with screening described as cursory or lax. These passengers were directed to airports including Los Angeles, San Francisco, New York, Chicago, Newark, Detroit and Seattle," says the team.

They further explain, "So, although our reconstructions incorporating unsampled lineages do not account for travel restrictions, the remaining influx likely provided ample opportunity for a second introduction to Washington State. It is also possible that the virus entered via nearby Vancouver, British Columbia, which is closely linked to both China and Washington state."

The study suggests that the form of the virus that spread in Washington arrived with Americans permitted to return to the US (Getty Images)

Missed opportunity 

The four weeks between WA1's arrival and the arrival of the actual source were a "missed opportunity" to stop the virus from taking hold in the US, say the authors. The researchers argue that the relatively late emergence of the outbreak implies that more American lives could have been saved by early action, such as testing and contact tracing. "Our analyses reveal an extended period of missed opportunity when intensive testing and contact tracing could have prevented SARS-CoV-2 from becoming established in the US," says the team.

The researchers explain that January and February 2020 were pivotal months as government officials tried to understand and appropriately respond to the unfolding SARS-CoV-2 emergency, amid considerable scientific uncertainty. When a new case was confirmed in a city that was not directly associated with travel, it was difficult to ascertain how the virus had gotten there and whether there had already been community transmission, say experts.

There was also considerable uncertainty about whether epidemiological contact tracing and isolation would be effective for controlling new outbreaks. “Conclusions that the Seattle area was already six weeks into an epidemic by the end of February, rather than two or three, and the notion that stringent efforts to prevent spread had failed in the WA1 case, may have influenced decision-making about how to respond to the outbreak, including whether such measures were worth the effort

Trevor Bedford, a scientist at the Fred Hutchinson Cancer Research Center, who had conducted the earlier study, said that his original assessment was incorrect. "On February 29, using the genetic sequence of the first community case of Covid-19 in Washington state, I made the claim that the Washington State outbreak descended directly from the January 15 arrival of the WA1 case with direct travel history to Wuhan. Based on data that has emerged in the intervening months, I no longer believe that a direct WA1 introduction is a likely hypothesis for the origin of the Washington state outbreak," he said.

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