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On the Diamond Princess 74% (70-78%) of infections proceeded asymptomatically, i.e. a 1:3.8 case-to-infection ratio. Despite the intense testing 53%, (51-56%) of infections remained undetected, most of them asymptomatic. Asymptomatic individuals were the source for 69% (20-85%) of all infections. (CMMID)


https://cmmid.github.io/topics/covid19/asymp-transmission.html

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see also https://www.theguardian.com/world/2020/may/30/could-nearly-half-of-those-with-covid-19-have-no-idea-they-are-infected

assume >50% asymptomatic
 and then remember those that do go on to show symptoms have already past peak transmissibility density

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a colleague from Cyprus tells me

in Cyprus where more tests are done by percentage of population,   80 % are asymptomatic 15 % don't need hospitalisation 5 percent need hospitalisation less than 2% die .

• with more routine testing on the general population  the percentage of asymptomatic cases goes up.

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Perhaps of more practical importance right now is a study that followed up 37 asymptomatic people who had positive PCR tests. This showed they had detectable levels of the virus for longer that those who had symptoms. Asymptomatic people are therefore likely to be more contagious. We also know that pre-symptomatic people, those in the early days following infection with Sars-CoV-2, are also highly contagious. Because we are often not aware that we are infected, measures to ensure social distancing and face covering are crucial when it comes to protecting others.

https://www.theguardian.com/commentisfree/2020/jun/25/viral-immunologist-antibody-tests-covid-19-immuity-coronavirus

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the study referred to above is in Nature

https://www.nature.com/articles/s41591-020-0965-6#Sec9

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updated transmission chart

the Nature curve referenced above has now apparently been updated

see attachment below & is still the reason why Covid has such a high R number

in answer to someone:
People's immune systems differ & while someone may not react symptomatically their viral load (density) can be & is sufficient for spread