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.