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Coast-to-Coast Spread of SARS-CoV-2 during the Early Epidemic in the United States

Our take —

This study analyzed SARS-CoV-2 sequences isolated from individuals in different cities in Connecticut, together with other sequences publicly available at that time (early March 2020). The authors investigated whether domestic or international travel was responsible for driving the early COVID-19 outbreak in Connecticut, and found that it was a result of multiple domestic introductions within the United States, and that international travel restrictions were likely not effective in curtailing the spread of the virus. To this effect, the authors explain the need for strengthening local surveillance systems in order to catch and reduce interstate introductions of the virus.

Study design


Study population and setting

The study analyzed 9 SARS-CoV-2 virus sequences collected in Connecticut together with 158 previously published sequences. The purpose of the study was to describe the roles of domestic and international virus spread in the emergence of new United States COVID-19 outbreaks.

Summary of Main Findings

The study analyzed SARS-CoV-2 sequences from the first 9 cases (March 6 to 14, 2020) identified in residents of eight different cities in Connecticut, none of whom had any international travel. Phylogenetic analyses showed that the outbreak in Connecticut was caused by multiple virus introductions (as evidenced by the virus sequences falling into three distinct clusters), and that most of these viruses were related to those sequenced from other states rather than international locations. This means that the transmissions in Connecticut were part of domestic transmission chains that resulted from undetected introductions. Using airline daily travel data, the researchers also showed that Connecticut was at a greater risk of domestic importation of COVID-19 rather than international viral importation, and concluded that restrictions on international travel to the United States may have not had a significant effect on reducing transmission.

Study Strengths

The researchers supported their phylogenetic findings of the greater role of domestic travel in fueling the pandemic by also performing modeling studies using airline daily travel. The methodology is clear and very detailed, and the code used for the analyses is provided by the authors. The authors also clearly outline the strengths and limitations of their analysis and provide concrete recommendations about control measures supported by their findings.


The conclusion that domestic, rather than international, importations played an important role in the early spread of COVID-19 in Connecticut is based on only nine genomes. Given this small number of sequences, and limited availability of other sequences from the United States to compare to, the title indicating “coast-to-coast spread” is potentially misleading. The authors also do not mention the role of the lack of diversity in the genomes in providing sufficient phylogenetic signal for the study.

Value added

The paper highlights gaps in implementing preventative measures against COVID-19 in the United States. Specifically, the authors show the greater contribution of domestic travel (versus international travel) in spreading the virus, and comment on the lack of domestic travel restrictions.