This article published in Science, the Journal of the American Association for the Advancement of Science seems to be the source of the statistic that 80% of infections come from asymptomatic cases.
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https://science.sciencemag.org/content/early/2020/03/24/science.abb3221
Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2)
Ruiyun Li1,*, Sen Pei2,*,†, Bin Chen3,*, Yimeng Song4, Tao Zhang5, Wan Yang6, Jeffrey Shaman2,†
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Science 16 Mar 2020:
eabb3221
DOI: 10.1126/science.abb3221
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"undocumented infections were the infection source for 79% of documented cases "
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"These undocumented infections often experience mild, limited or no symptoms and hence go unrecognized "
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"Here we use observations of reported infection within China, in conjunction with mobility data, a networked dynamic metapopulation model and Bayesian inference, to infer critical epidemiological characteristics associated with SARS-CoV2, including the fraction of undocumented infections and their contagiousness. We estimate 86% of all infections were undocumented (95% CI: [82%–90%]) prior to 23 January 2020 travel restrictions. "
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