Abstract
Evidence-based public health approaches that minimize the introduction and spread of new SARS-CoV-2 transmission clusters are urgently needed in the United States and other countries struggling with expanding epidemics. Here we analyze 247 full-genome SARS-CoV-2 sequences from two nearby communities in Wisconsin, USA, and find surprisingly distinct patterns of viral spread. Dane County had the 12th known introduction of SARS-CoV-2 in the United States, but this did not lead to descendant community spread. Instead, the Dane County outbreak was seeded by multiple later introductions, followed by limited community spread. In contrast, relatively few introductions in Milwaukee County led to extensive community spread. We present evidence for reduced viral spread in both counties following the statewide “Safer at Home” order, which went into effect 25 March 2020. Our results suggest patterns of SARS-CoV-2 transmission may vary substantially even in nearby communities. Understanding these local patterns will enable better targeting of public health interventions.
Highlights
Evidence-based public health approaches that minimize the introduction and spread of new SARS-CoV-2 transmission clusters are urgently needed in the United States and other countries struggling with expanding epidemics
Our analyses suggest that transmission between Dane and Milwaukee counties has not been a principal component of viral spread within either region
Wisconsin had one of the earliest detected cases of SARS-CoV-2 infection in the United States, but this infection did not spark community spread. This is probably due to a combination of good infection control practices by healthcare providers, the patient, and sheer luck
Summary
Evidence-based public health approaches that minimize the introduction and spread of new SARS-CoV-2 transmission clusters are urgently needed in the United States and other countries struggling with expanding epidemics. Patterns of viral sequence variation can be used to estimate epidemiological parameters, including the total number of infections in a given population and epidemic doubling time, independent of case counts[4,7,8,9,10,11,12,13,14,15,16] We apply these methods to gain a nuanced view of SARS-CoV-2 transmission within and between regions of the American Upper Midwest. From the start of the Executive Order through 21 April 2020, Dane and Milwaukee counties had the highest documented number of SARS-CoV-2 cases in Wisconsin These two counties provide a “natural experiment” to understand the impact of the “Safer at Home” Executive Order on within- and betweencounty SARS-CoV-2 transmission in two US counties with distinguishing demographic features
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