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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 (https://creativecommons.org/licenses/by/4.0/).
(a) Air quality (PM2.5), (b) the total number of COVID-19 cases per 100,000 people, (c) GDP per capita, and (d) the total days of lockdown of 158 countries worldwide in 2020. Note: The darker (lighter) the color, the higher (lower) the value of the indicator.