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The emergence of SARS-CoV-2 in Europe and the US

Michael WorobeyiD, Jonathan E. PekariD, Brendan B. LarseniD, Martha I. NelsoniD, Verity HilliD, Jeffrey B. JoyiD, Andrew RambautiD, Marc A. SuchardiD, Joel O. WertheimiD, Philippe LemeyiD

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Abstract

Accurate understanding of the global spread of emerging viruses is critically important for public health response and for anticipating and preventing future outbreaks. Here, we elucidate when, where and how the earliest sustained SARS-CoV-2 transmission networks became established in Europe and the United States (US). Our results refute prior findings erroneously linking cases in January 2020 with outbreaks that occurred weeks later. Instead, rapid interventions successfully prevented onward transmission of those early cases in Germany and Washington State. Other, later introductions of the virus from China to both Italy and Washington State founded the earliest sustained European and US transmission networks. 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 and Europe.

Abstract summary

Claude Opus 5.5 · xhigh effort · 18 Oct 2026 · from the abstract · CC BY-NC-ND 4.0

January 2020 cases in Germany and Washington State were contained, while separate later introductions from China seeded the first sustained SARS-CoV-2 transmission networks in Italy and Washington State.

Which introductions actually founded sustained transmission decides whether early containment worked and where surveillance failed. The authors say earlier work wrongly connected January 2020 cases to outbreaks appearing weeks afterwards, and they reassign the origin of the first European and US transmission chains.

Key results

  1. ClaimedThe January 2020 cases detected in Germany and Washington State did not give rise to the later outbreaks there; the authors attribute this to interventions that stopped onward spread.

    Presented in the abstract as a refutation of prior findings, without case counts, dates, sequence numbers or statistical support in the available text. · Abstract

  2. ClaimedThe earliest sustained European and US transmission networks trace instead to separate, later introductions of the virus from China into Italy and into Washington State.

    Stated as the study's main conclusion; the abstract reports no timings, no genomic data and no measure of uncertainty for these introduction events. · Abstract

  3. ClaimedA prolonged window is identified in which intensive testing and contact tracing could have kept the virus from becoming established in Europe and the US.

    A counterfactual conclusion asserted in the closing sentence; its duration is not quantified and no supporting analysis is described in the text. · Abstract

Methods, data and reproducibility

The abstract does not describe the analytical approach. It states only that analyses were run to date and locate the first self-sustaining chains of SARS-CoV-2 spread in Europe and the United States, and to trace where each chain was introduced from, with the conclusions set against earlier reports that tied January 2020 cases to outbreaks detected weeks afterwards.

Limitations (2)
  • No data, sampling dates, sequence counts, models or uncertainty estimates appear in the available text, so none of the reassignments of outbreak origin can be checked here. · noted in this summary
  • The missed-opportunity conclusion is a counterfactual about unobserved interventions, and the abstract neither bounds the period involved nor states how robust it is to sampling gaps. · noted in this summary

Read this if you reconstruct outbreak origins and want the authors' revised account of which early SARS-CoV-2 introductions did and did not found sustained transmission in Europe and the US.

Citations
57

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