How did COVID-19 actually spread through North America? New study says the early cases in Washington state were not the culprit (Details)


In mid-February, all eyes were on the West Coast — just one month after patient WA1 became the first person to test positive for COVID-19 on American soil after landing in Seattle from Wuhan, China.

Washington state is where the continent’s first COVID-19 outbreaks occurred and it was widely presumed it would spread eastward from there like wildfire.

But a new study reveals that the first documented infected individuals who arrived from China into the western United States and Canada were not the source of widespread continental outbreaks. Instead, it is believed the virus was primarily brought by travellers flying from China through Europe and into the eastern United States, before spreading across the whole continent.

The study, published last week and partly funded by the Canadian Institutes of Health Research, paints a picture of how the West Coast handled the early stages of the pandemic compared to the East, with dramatically different results. It shows that while attention was initially focused on patient WA1, Washington and the West Coast, what was happening there was merely a flicker compared to the inferno igniting in the East.

“We kind of hoped for those next six weeks (from mid-January) until the end of February that maybe we’re catching all of the sparks that are landing and maybe we’re OK,” said University of Arizona researcher Michael Worobey, who led an interdisciplinary team of scientists from multiple countries, including Canada, for the study.

“But it turned out that what was happening was sparks were catching fire throughout February, in various places, including the East Coast.”

The researchers used genome sequencing, which compares differences in viral variants, as well as computer simulations and travel records to determine if various samples of the virus are epidemiologically linked.

Worobey said the United States was largely “flying blind” for that six-week period between mid-January and late February, when COVID-19 was silently but steadily spreading.

It wasn’t until late February when more cases starting popping up that were not connected to patient WA1.

“People started thinking ‘Holy crap, if this has been going since this guy arrived January 15, thousands of people are infected with this thing. What’s going to happen now?’”

For the first quarter of 2020, it was believed that patient WA1 infected someone and established a cluster of cases that silently spread for six weeks. But genome sampling from other cases that emerged in Washington show the samples were different enough that it was “very unlikely” patient WA1 was the source of the outbreak.

Instead, the researchers believe the source of the large outbreaks most likely arrived from a direct flight from China on or around Feb. 1. Even though the U.S. was prohibiting Chinese citizens from entering the country, they were still allowing U.S. citizens and visa holders from China to return, which amounted to some 40,000 people, Worobey said.

And while the first outbreaks originated in Washington, the state was effective in issuing early warnings to the public, instating a community testing program, putting limits on gatherings and shutting down the University of Washington.

Meanwhile, it was mostly business as usual in the East and flights from China and Europe were still coming into New York en masse.

“Even though the epidemic actually kind of got started with one of these dispersals from China to the West Coast, it was the East Coast that was that was really getting hammered and then finding fertile soil to spread,” Worobey said.

“If only we could have copied the effort in Seattle … it’s not impossible that this outbreak could have been prevented in North America, just like it largely has been in places like South Korea,” he added.

On Jan. 25, the first presumptive case was detected in Toronto. In February, several cases were detected in travellers returning from abroad, including Iran and Egypt. By mid-March, community transmission was prevalent, and Premier Doug Ford declared a state of emergency on March 17.

Another major discovery in the study is that the virus did not spread from China, into B.C., then into Washington state, as was widely presumed, but rather the other way around.

Multiple COVID-19 genomes published by the British Columbia Centre for Disease Control appeared to be “ancestral” to the viral samples tested in Washington state, which would point to a Canadian origin. However, that turned out to be a testing mistake.

“It’s clear that this virus actually moved into Washington state and then up to up to B.C. and not vice versa,” Worobey said.

The researchers conclude that inadequate early testing, especially of those travellers coming from China into the U.S., as well as a lack of contact tracing and enforcement of physical distancing helped facilitate the spread of the virus.

“The long and short of it is, when we had a chance maybe to catch these embers when they were first landing on North American soil and detect these chains of transmission in the community, we didn’t take that opportunity,” Worobey said.

The primary takeaway is that tools such as testing, contact tracing and physical isolation can make a huge difference in preventing large-scale outbreaks — if implemented early enough.

Furthermore, it shows that one individual, like patient WA1, can potentially change the course of history, Worobey said.

“I think there is something really nice here where we have this guy who could have gone down in history as the person who started a pandemic … Our research clarifies that’s probably not what happened. And in fact, his behaviour is actually a good model for all of us moving forward to take this seriously, do the responsible thing and prevent onward transmission.”


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