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How COVID-19 Took Hold in North America and Europe

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SARS-CoV-2 Tracking
Caption: SARS-CoV-2 introductions to U.S. and Europe. Credit: Modified from Worobey M, Science, 2020.

It was nearly 10 months ago on January 15 that a traveler returned home to the Seattle area after visiting family in Wuhan, China. A few days later, he started feeling poorly and became the first laboratory-confirmed case of coronavirus disease 2019 (COVID-19) in the United States. The rest is history.

However, new evidence published in the journal Science suggests that this first COVID-19 case on the West Coast didn’t snowball into the current epidemic. Instead, while public health officials in Washington state worked tirelessly and ultimately succeeded in containing its sustained transmission, the novel coronavirus slipped in via another individual about two weeks later, around the beginning of February.

COVID-19 is caused by the novel coronavirus SARS-CoV-2. Last winter, researchers sequenced the genetic material from the SARS-CoV-2 that was isolated from the returned Seattle traveler. While contact tracing didn’t identify any spread of this particular virus, dubbed WA1, questions arose when a genetically similar virus known as WA2 turned up in Washington state. Not long after, WA2-like viruses then appeared in California; British Columbia, Canada; and eventually 3,000 miles away in Connecticut. By mid-March, this WA2 cluster accounted for the vast majority—85 percent—of the cases in Washington state.

But was it possible that the WA2 cluster is a direct descendent of WA1? Did WA1 cause an unnoticed chain of transmission over several weeks, making the Seattle the epicenter of the outbreak in North America?

To answer those questions and others from around the globe, Michael Worobey, University of Arizona, Tucson, and his colleagues drew on multiple sources of information. These included data peretaining to viral genomes, airline passenger flow, and disease incidence in China’s Hubei Province and other places that likely would have influenced the probability that infected travelers were moving the virus around the globe. Based on all the evidence, the researchers simulated the outbreak more than 1,000 times on a computer over a two-month period, beginning on January 15 and assuming the epidemic started with WA1. And, not once did any of their simulated outbreaks match up to the actual genome data.

Those findings suggest to the researchers that the idea WA1 is responsible for all that came later is exceedingly unlikely. The evidence and simulations also appear to rule out the notion that the earliest cases in Washington state entered the United States by way of Canada. A deep dive into the data suggests a more likely scenario is that the outbreak was set off by one or more introductions of genetically similar viruses from China to the West Coast. Though we still don’t know exactly where, the Seattle area is the most likely site given the large number of WA2-like viruses sampled there.

Worobey’s team conducted a second analysis of the outbreak in Europe, and those simulations paint a similar picture to the one in the United States. The researchers conclude that the first known case of COVID-19 in Europe, arriving in Germany on January 20, led to a relatively small number of cases before being stamped out by aggressive testing and contact tracing efforts. That small, early outbreak probably didn’t spark the later one in Northern Italy, which eventually spread to the United States.

Their findings also show that the chain of transmission from China to Italy to New York City sparked outbreaks on the East Coast slightly later in February than those that spread from China directly to Washington state. It confirms that the Seattle outbreak was indeed the first, predating others on the East Coast and in California.

The findings in this report are yet another reminder of the value of integrating genome surveillance together with other sources of data when it comes to understanding, tracking, and containing the spread of COVID-19. They also show that swift and decisive public health measures to contain the virus worked when SARS-CoV-2 first entered the United States and Europe, and can now serve as models of containment.

Since the suffering and death from this pandemic continues in the United States, this historical reconstruction from early in 2020 is one more reminder that all of us have the opportunity and the responsibility to try to limit further spread. Wear your mask when you are outside the home; maintain physical distancing; wash your hands frequently; and don’t congregate indoors, where the risks are greatest. These lessons will enable us to better anticipate, prevent, and respond to additional outbreaks of COVID-19 or any other novel viruses that may arise in the future.


[1] The emergence of SARS-CoV-2 in Europe and North America. Worobey M, Pekar J, Larsen BB, Nelson MI, Hill V, Joy JB, Rambaut A, Suchard MA, Wertheim JO, Lemey P. Science. 2020 Sep 10:eabc8169 [Epub ahead of print]


Coronavirus (COVID-19) (NIH)

Michael Worobey (University of Arizona, Tucson)

NIH Support: National Institute of Allergy and Infectious Diseases; Fogarty International Center; National Library of Medicine


  • Sugathadasa R says:

    In my humble opinion the main reason for this pandemic to be so severe in Western Nations is mainly lack of Mindfulness. Mindful people are always in the present moment. They do not digress to dead past or the unborn future. Living in the present moment knowing how virulent this virus could be they take precautions to ensure it will not take hold. The precautions were already there, wearing face mask and keeping social distance. Images that I saw on western TV channels in beach and parks showed how people were hugging each other and not keeping distance.
    That was the cause for this effect of so many patients and deaths. Mindfulness can be developed with Mindfulness Meditation and that also increase the human capability to overcome these situations with more immunity.

  • Haresh Chunilal Patel says:

    Not recognizing the airborne transmission of COVID-19 is the cause of widespread infections. Countries which have recognized this mode have negligible infection rates without spending much money. Most underdeveloped countries have order of magnitude lower infection rates because they have few central HVAC systems and most of their facilities have open ventilation although they are not actually aware of the natural benefit.


    I’m curious about it spreading by Humane Waste and Waste Water here in WI we have Septic Companies spreading it on farm fields and growing crops. It’s been found in Waste waters tested and living there up to two weeks. Human wastes is being used as fertilizer untreated waste spread on farm fields and grow crops in it. We have large pits as holding tanks full of Human Poop Sitting in fields close to ground water, well water, rivers, and Lakes. Also steams that wildlife live in and are found sick and dying. How do we get test done to find Covid in our Waters I live in WI and our numbers are very High here Now and I read that Waste Water was tested in other States and it was found in it. I’m worried it makes me think it’s possible. Human Waste is a Biohazard and it is being spread all around us

  • Rod Krieger says:

    While studying the four fundamental forces in the beginning of my physics education, my professor wrote the word “gravity” on the blackboard and said “this word represents a lot of ignorance”. Every time I see the term “COVID-19” my mind takes me back to that moment.

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