Background The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has
caused global disruption of human health and activity. Being able to trace the early outbreak
of SARS-CoV-2 within a locality can inform public health measures and provide insights to
contain or prevent viral transmission. Investigation of the transmission history requires
efficient sequencing methods and analytic strategies, which can be generally useful in the
study of viral outbreaks. Methods The County of Los Angeles (hereafter, LA County) …

Abstract Los Angeles (LA) County has sustained a large outbreak of severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2). To learn about the transmission history
of SARS-CoV-2 in LA County, we sequenced 142 viral genomes from unique patients
seeking care at UCLA Health System. 86 of these genomes are from samples collected
before April 19, 2020. We found that the early outbreak in LA, as in other international air
travel hubs, was seeded by multiple introductions of strains from Asia and Europe. We …