The Italian number (75%) comes from testing everyone in a village of 3000 people. The Chinese number (1%) comes from testing everyone who comes to hospitals and maybe their family members. I would guess that the Chinese sample was selected for people displaying symptoms (asymptomatic people will not go to the clinic), while the Italian number biases for people early in the course of the disease. For the purposes of public health and controlling disease spread, the Italian number is more meaningful.

Note also that it is possible the virus has evolved to spread more easily, which in the context of a lot of screening and social distancing of symptomatic people would mean more asymptomatic spreading.

How are the South Korean's coming up with such few positives if the disease is presumably spreading among the 70% with no symptoms?

Most "asymptomatic" people will show symptoms within two weeks, so aggressive cluster/contact tracing works. South Korean local goveenment staff are following the credit card use, transit use, and other daily movements (CCTV logs) for every confirmed case, then issue two week self-isolation orders to those who shared a workplace, restaurant, cafe, church, or elevator with the confirmed case. Since most cases get a short lived fever or cough within two weeks, SK has been able to stop most transmission chains. I'm actually a bit confused about the scope of SK testing. It looks like everyone with symptoms is being tested, as well as quarantined health care workers and quarantined people from high risk groups, but I'm not sure whether asymptomatic people are tested at the end of their quarantine period.

Yup. Apparent efficacy of the South Korean approach versus more generalised 'self-isolate if you have symptoms' instructions would be a point in favour of the disease being often asymptomatic [for extended periods]

They wore medical grade mask in the first place. Hygiene and cautious about it. They stopped and block visits from China ( or any where else with COVID ) very early on. Instead of acting everything is going to be great despite all the warnings, they were mindful and kept social gathering to minimal. ( Comparatively speaking ). Citizens were also proactively reporting if they were having symptoms, and government are testing every case as well as tracing where they have been and issue warning.

Medical Mask also meant even though the case broke out in a city with very high population density, the spread is far lower ( again comparatively speaking ).

Where did you get the information that they've blocked people traveling from anywhere with Covid. You can still turn up now from Europe and be let in, they just ask you to install an app and report your health status daily.

Early strict containment and disciplin.

Thanks, makes sense. Iceland also did voluntary testing and then estimated 1% of their population is infected. I doubt 1% is even showing mild symptoms. Remains the significance of asymptomatic spread. If someone does not cough or sneeze, are they shedding enough to be a formidable source of contagion?

https://grapevine.is/news/2020/03/15/first-results-of-genera...

Are there countries who conduct serologic tests? PCR tests (widely used?) wouldn't catch people who got the infection, developed antibodies with no or mild symptoms, but didn't transmit the virus around at the moment test is taken.

From the article about Covid-19 serologic test development:

> The serologic tests, which are different from the ones used to diagnose active infection, would allow researchers to test the blood of people who were not confirmed cases of Covid-19 in communities where the virus spread. They would be designed to look for signs that people have mounted an immune response after being exposed to the virus.

https://www.statnews.com/2020/03/11/cdc-developing-serologic...

> Are there countries who conduct serologic tests? PCR tests (widely used?) wouldn't catch people who got the infection, developed antibodies with no or mild symptoms, but didn't transmit the virus around at the moment test is taken.

Not that I know of. On the most recent "This Week in Virology" podcast, they mentioned that China has recently said they won't be doing serologic testing, which lead them to think that China has reason to believe the disease is far more widespread than their official numbers show. This may be in line with the idea that most infections are asymptomatic.

Another source:

> "The current PCR test only tells you whether a person has the virus at this moment," Dr Meru Sheel, an epidemiologist and research fellow at the Australian National University said.

> "What it doesn't tell you is that a person may have had the infection, has recovered and is immune or not immune."

> Dr Sheel said a serological test would help understand at population level how many people have been exposed to the disease, have recovered and if there are populations that are not immune to the virus.

https://www.abc.net.au/news/2020-03-17/coronavirus-experts-c...

> "The vast majority of people infected with Covid-19, between 50 and 75%, are completely asymptomatic but represent a formidable source of contagion"

So long as every asymptomatic person got it from someone with symptoms, they necessarily a source of contagion. Is the conclusion based on modeling, i.e. that the rate of spread is only consistent with the asymptomatic people also trasmitting the virus?

Lots of asymptomatic infected doesn't (automatically) mean lots of transmission from asymptomatic people to uninfected people.

Well they aren't testing symptomatic people. How would they know to want to be tested other than personal election to do so because they had been in contacted with someone else?