Using molecular dating tools and epidemiological simulations, researchers at the University of California (UC) San Diego School of Medicine, with colleagues at the University of Arizona and Illumina, Inc., estimate that the SARS-CoV-2 virus was likely circulating undetected for at most two months before the first human cases of COVID-19 were described in Wuhan, China in late-December 2019. In an article published online on March 18, 2021 in Science, the scientists also note that their simulations suggest that the mutating virus dies out naturally more than three-quarters of the time without causing an epidemic. The open-access article is titled “Timing the SARS-CoV-2 Index Case in Hubei Province.” (https://science.sciencemag.org/content/early/2021/03/17/science.abf8003). "Our study was designed to answer the question of how long could SARS-CoV-2 have circulated in China before it was discovered," said senior author Joel O. Wertheim, PhD, Associate Professor in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine. "To answer this question, we combined three important pieces of information: a detailed understanding of SARS-CoV-2 spread in Wuhan before the lockdown, the genetic diversity of the virus in China, and reports of the earliest cases of COVID-19 in China. By combining these disparate lines of evidence, we were able to put an upper limit of mid-October 2019 for when SARS-CoV-2 started circulating in Hubei province." Cases of COVID-19 were first reported in late-December 2019 in Wuhan, located in the Hubei province of central China. The virus quickly spread beyond Hubei. Chinese authorities cordoned off the region and implemented mitigation measures nationwide.
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