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. 2021 Jun 26;18(13):6858. doi: 10.3390/ijerph18136858

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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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The contour of reproduction number $R_{e}$ with different $η$ and $χ$ .