SARS-CoV-2 emergence very likely resulted from at least two zoonotic events

Understanding the circumstances that lead to pandemics is critical to their prevention. Here, we analyze the pattern and origin of genomic diversity of SARS-CoV-2 early in the COVID-19 pandemic. We show that the SARS-CoV-2 genomic diversity prior to February 2020 comprised only two distinct viral lineages—denoted A and B—with no transitional haplotypes. Novel phylodynamic rooting methods, coupled with epidemic simulations, indicate that these two lineages were the result of at least two separate cross-species transmission events into humans. The first zoonotic transmission likely involved lineage B viruses and occurred in late-November/early-December 2019 and no earlier than the beginning of November 2019, while the introduction of lineage A likely occurred within weeks of the first event. These findings define the narrow window between when SARS-CoV-2 first jumped into humans and when the first cases of COVID-19 were reported. Hence, as with SARS-CoV-1 in 2002 and 2003, SARS-CoV-2 emergence likely resulted from multiple zoonotic events.