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Cryptic transmission of SARS-CoV-2 in Washington State

Our take —

This study uses the genetic sequence of SARS-CoV-2 viruses to determine that COVID-19 cases in Washington state, USA in early March 2020, were the result of a single introduction of the virus, and that this introduction was likely 4-6 weeks before COVID-19 was first detected in the United States. This finding underscores the importance of testing and virus sequencing to understand the spread of SARS-CoV-2. Specifically, this paper shows that virus sequencing is an important tool for distinguishing between local transmission and imported cases, which can and will play an important role in determining the most effective containment measures to implement at different stages of the outbreak.

Study design


Study population and setting

This study involved the analysis of 346 SARS-CoV-2 genetic sequences from Washington state. These genomes are from infected patients in Washington State, USA between February 20 and March 15, 2020.

Summary of Main Findings

Analysis of virus sequence data from Washington state, USA shows that sequences from February/early March 2020 in this region are highly similar to each other, which suggests that most of these cases were from a single introduction of the virus from Wuhan, China. A single introduction implies community transmission in the area before widespread testing and surveillance were implemented. The authors also estimate that the virus was circulating in Washington state for 4-6 weeks before it was first detected. Both of these findings highlight the importance of testing and genomic sequencing to understand the spread of the virus and detect community transmission, even after the pandemic is under better control.

Study Strengths

This study uses a relatively large number of sequences from a distinct time and place. This dense sampling increases the probability of drawing correct conclusions that will be (and since have been) supported by additional sequencing studies. Additionally, the findings in this study are presented with appropriate caveats about what can and cannot be definitively inferred from this type of data. The study also connects the scientific findings to concrete public health measures (i.e., the importance of continued SARS-CoV-2 surveillance at different stages of the outbreak).


The study is limited in its scope because all genomes are from a particular part of the county and from early in the outbreak, when limited sequencing data from other regions was available. Additionally, as noted by the authors, only a small proportion of infections from China were sequenced at the time of publication, which made it difficult to determine exactly when community transmission in Washington state began.

Value added

This study is a good example of how virus sequencing data can be used to distinguish between cases imported from travelers and local, community transmission, which is of key importance when determining the type of containment measures to take (e.g., travel bans versus stay-at-home orders). The study demonstrates that community transmission was likely occurring in Washington state even before the first case was detected, and thereby highlights the importance of this type of surveillance for understanding the spread of COVID-19.