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Interregional SARS-CoV-2 spread from a single introduction outbreak in a meat-packing plant in northeast Iowa

Our take —

This study, available as a preprint and thus not yet peer reviewed, describes sequencing and phylogenetic analysis of SARS-CoV-2 from cases identified by the Gundersen Healthcare System, an integrated healthcare system serving parts of Wisconsin, Iowa, and Minnesota, USA. The authors identify several introductions of the virus into the region, most of which did not result in sustained transmission chains. The exception to this was a virus lineage linked to a meat processing plant in Iowa, which ultimately spread to seven counties in three different states. This study clearly shows the spread of a virus lineage from its source, and highlights how early detection of these transmission chains can be used to improve current disease control strategies.

Study design


Study population and setting

The study generated and analyzed 67 SARS-CoV-2 virus sequences collected from COVID-19 cases diagnosed between March 18 and May 24,2020 in selected counties from three states (Wisconsin, Iowa and Minnesota). The study monitored the introduction and spread of SARS-CoV-2 to the service area of the Gundersen Health System (La Crosse, Wisconsin) which provides care in 21 counties in southwestern Wisconsin, northeastern Iowa and southeastern Minnesota.

Summary of Main Findings

The authors compared the 67 sequences from the Gundersen Health System to published sequences from other places in Wisconsin, Minnesota, Iowa, and Illinois, and found that there were 14 independent SARS-CoV-2 introductions (distinct virus sequences) into the health system region. In most cases, all sequences from the same virus introduction were found in individuals from the same household, indicating that these introductions did not result in sustained transmission chains. The exception was a cluster of 27 cases starting in Allamakee County associated with a meatpacking plant in Postville, Iowa, which spread to at least seven counties spanning three states. Early cases of this strain were among households connected to the meat packing factory, and later on, cases were detected among individuals with no known direct link to the meatpacking workforce. The authors also report high variability in symptoms presented by 27 individuals affected with the outbreak strain.

Study Strengths

The study used genetic sequencing to trace an outbreak originating from a meatpacking plant and compares variability in clinical presentation by individuals in this cluster. The researchers clearly demonstrate how this single virus lineage spread through time and space, highlighting the utility of molecular and phylogenetic analyses in understanding viral spread.


Due to low sample sizes, comparison of symptoms and other clinical metadata with the viral sequence data is limited.

Value added

Among the 14 SARS-CoV-2 sequences introduced into this population, most did not result in sustained transmission chains. However, one introduction caused a cluster of COVID-19 cases in a meatpacking facility, and this study the spread of this virus to surrounding counties using molecular and phylogenetic analyses. The authors demonstrate the importance of early public health interventions to contain the spread of the virus in meat packing facilities and other industrial settings. The study also contributes to the growing body of research showing that individuals with the same virus strain may experience a wide range of clinical symptoms.

This review was posted on: 10 July 2020