This paper outlines that COVID-19 circulated cryptically and was undetected 4-6 weeks before the first community transmission case was detected at Seattle. Mass testing will enable public health officials to detect and track these under-detected community cases.

Following its emergence in Wuhan, China, in late November or early December 2019, the SARS-CoV-2 virus has rapidly spread throughout the world. On March 11, 2020, the World Health Organization declared Coronavirus Disease 2019 (COVID-19) a pandemic.

Genome sequencing of SARS-CoV-2 strains allows for the reconstruction of transmission history connecting these infections. This study presents an analysis of 346 SARS-CoV-2 genomes from samples collected between 20 February and 15 March 2020 from infected patients in Washington State, USA.

The authors found that the large majority of SARS-CoV-2 infections sampled during this time frame appeared to have derived from a single introduction event into the state in late January or early February 2020 and subsequent local spread. This strongly suggests cryptic spread of COVID-19 during the months of January and February 2020, before active community surveillance was implemented.

We estimate a common ancestor of this outbreak as occurring between 18 January and 9 February 2020.

From genomic data, we estimate an exponential doubling between 2.4 and 5.1 days.

These results highlight the need for large-scale community surveillance for SARS-CoV-2 introductions and spread and the power of pathogen genomics to inform epidemiological understanding.