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. 2021 May 12;18(10):5139. doi: 10.3390/ijerph18105139

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© 2021 by the authors.

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/).

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Study design with critical time frames. ¹ Puls Medycyny, Pierwszy, potwierdzony przypadek COVID-19 w Polsce, https://pulsmedycyny.pl/pierwszy-potwierdzony-przypadek-covid-19-w-polsce-984079 (accessed on 30 November 2020). First confirmed case of COVID-19 in Poland. ² Premier: Wprowadzamy stan epidemii w Polsce, https://www.gov.pl/web/premier/premier-wprowadzamy-stan-epidemii-w-polsce (accessed on 30 November 2020). The Prime Minister’s government imposed a state of epidemic lockdown in Poland.