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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(a) Distribution of urban density, (b) averaged urban density by county, (c) cumulative cases on COVID-19 at the county level, (d) COVID-19 infected proportion at county level over the United States.