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According to https://english.alarabiya.net/en/features/2020/03/25/Coronavirus-Iceland-s-mass-testing-finds-half-of-carriers-show-no-symptoms, half of the SARS-CoV-2 virus virus carriers show no symptoms.

What are the biological factors that determine whether an infected individual will be asymptomatic or not?

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    Do we have information about the specificity of the tests performed in Iceland? Keep in mind that we expect to see (1-specificity)* ntests false positives. And those should be both positive and without symptoms. The validation data for the tests I've seen so far (at FDA EUA page) would be consistent with, say, 97% specificity. 3 % false positives of 12 k tests is 360, almost half the number of positive tests in Iceland. – cbeleites unhappy with SX Mar 26 '20 at 14:13
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    @not2qubit Your comment doesn't make any sense. The question is about which people are asymptomatic, not about guessing who has the disease. The article title is "half of carriers show no symptoms": that's exactly what this question is asking about. – Bryan Krause Mar 26 '20 at 21:02
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    @not2qubit No, the question can just be "How can one predict if an individual affected by covid-19 will be asymptomatic?" which is the one asked. Answers might include demographics, extent of exposure, etc. – Bryan Krause Mar 26 '20 at 21:50
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    i've edited your question to what I think you might be asking. – Graham Chiu Mar 27 '20 at 21:29
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    @Fizz sorry for the unclarity, I mean infected with the virus – Franck Dernoncourt Mar 27 '20 at 21:44
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This question can't be answered with any certainty due to the lack of published full data, or the conflicting data when it is being published.

When blanket testing was done of the isolated town of Vo’Eugeano, they reported a high percentage of those tested were asymptomatic. They gave no figures. And this contrasted with the original reports from the WHO in China (https://www.bmj.com/content/368/bmj.m1165)

“the proportion of truly asymptomatic infections is unclear but appears to be relatively rare and does not appear to be a major driver of transmission.”

So, for instance, did the Italians follow the asymptomatic until they cleared the virus? Did they check for emerging symptoms such as anosmia? Diarrhoea?

Our best guess is that the people who are going to be minimally symptomatic are those who are least affected with the disease, and this includes, older children ( but not the very young ), female, non-smokers/non-vaping, blood group O, and no existing medical co-morbidities such as hypertension, diabetes, cardiopulmonary disease.

This paper says 86% of Chinese had undocumented infection with a high probability of many of them being not very symptomatic.

We estimate 86% of all infections were undocumented (95% CI: [82%–90%]) prior to 23 January 2020 travel restrictions. Per person, the transmission rate of undocumented infections was 55% of documented infections ([46%–62%]), yet, due to their greater numbers, undocumented infections were the infection source for 79% of documented cases. These findings explain the rapid geographic spread of SARS-CoV2 and indicate containment of this virus will be particularly challenging.

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† 1 MRC Centre for Global Infectious Disease Analysis, Department of Infectious Disease Epidemiology, School of Public Health, Faculty of Medicine, Imperial College London, London W2 1PG, UK. 2 Department of Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, NY 10032, USA. 3 Department of Land, Air and Water Resources, University of California, Davis, Davis, CA 95616, USA. 4Department of Urban Planning and Design, The University of Hong Kong, Hong Kong. 5 Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University, Beijing 10084, P. R. China. 6Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY 10032, USA. https://science.sciencemag.org/content/early/2020/03/24/science.abb3221/tab-pdf

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    The question is about "who", not "how many". – Mark Mar 27 '20 at 20:13
  • the question is very unclear. What does who mean? – Graham Chiu Mar 27 '20 at 21:04
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    I find the question to be quite clear: given a population that has been infected by COVID-19, about half of those infected will show no symptoms. How can we predict which individuals make up that half? – Mark Mar 27 '20 at 21:13
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    I agree with Mark (and argued as such in the comments above), but there isn't a button to vote that the question is clear. – Bryan Krause Mar 27 '20 at 21:32
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    Your "86% asymptomatic" is wrong: the paper says that 86% of infections are undocumented. This includes both asymptomatic infections and infections that were symptomatic but not recorded. – Mark Mar 28 '20 at 3:17

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