Abstract
Accurate understanding of the global spread of emerging viruses is critical for public health responses and for anticipating and preventing future outbreaks. Here we elucidate when, where, and how the earliest sustained severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission networks became established in Europe and North America. Our results suggest that rapid early interventions successfully prevented early introductions of the virus from taking hold in Germany and the United States. Other, later introductions of the virus from China to both Italy and Washington state, United States, founded the earliest sustained European and North America transmission networks. Our analyses demonstrate the effectiveness of public health measures in preventing onward transmission and show that intensive testing and contact tracing could have prevented SARS-CoV-2 outbreaks from becoming established in these regions.
Highlights
In late 2019 the emergence of SARS-CoV-2, which causes COVID-19, ignited a pandemic that has been associated with over 500,000 deaths globally as of July, 2020
Subsequent identification of viruses that were genetically similar to WA1, first in Washington, in Connecticut [3], California [5], British Columbia [6] and elsewhere, raised the possibility that WA1 had established chains of cryptic transmission that started on January 15th and went undetected for several weeks [1, 2]
Our analyses delineate when widespread community transmission was first established on both continents www.sciencemag.org (Page numbers not final at time of first release) 5 (Fig. 6) and clarify the period before SARS-CoV-2 establishment when contact tracing and isolation might have been most effective
Summary
Even though a second introduction into Washington State (independent of WA1) implies a more recent date of origin of that transmission cluster than under the WA1-origin scenario (~February 1st versus January 15th, if it had originated with WA1), the WA outbreak clade still predates the earliest genomically-identified transmission clusters elsewhere in the US: the large one in NYC [4] plus two smaller, apparently self-limited clusters from California (in Solano County and Santa Clara County) that appear to have been introduced from China [5]. The transmission cluster from Santa Clara County appears older, dating to before February 22nd, 2020 (95% HPD February 5th–February 29th) (Supplementary text)
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