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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?
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
