[M T]

When the virus started to spread in Guangdong — the province where the 2002-2003 SARS outbreak began — worried people flooded fever clinics to be tested. Of 320,000 tests performed, just under 0.5% were positive for the virus at the peak of transmission there, he said — which suggests that only 1 case out of 200 was being missed.

That doesn't make sense or at least the phrasing is misleading.  If the 320,000 people tested were representative and .5% were positive, it doesn't mean that 1 in 200 cases are missed, it means that 1 in 200 people have an undetected case.  The population of Guandong is 113.46 million, so if .5% are positive, that's 56500 undetected cases.  Since there are 1350 cases in the province, that means that >97% of cases are undetected.

Good catch.  I took that quote from the article, not from the WHO-China Joint Mission report.  I shouldn't have trusted it.  The numbers are wrong and the interpretation in the article is faulty, as you pointed out.
From the report

Within the fever clinics in Guangdong, the percentage of samples that tested positive for the COVID-19 virus has decreased over time from a peak of 0.47% positive on 30 January to 0.02% on 16 February.  Overall in Guangdong, 0.14% of approximately 320,000 fever clinic screenings were positive for COVID-19.

At a "fever clinic", the patients will not be asymptomatic and will not be a random sample of the population.  0.14% translates to about 448 patients detected at the fever clinics. 

Interestingly, tests for COVID-19 (mostly in January) of samples of Influenza-like Illnesses had very few positive hits. The highest reported was 3 out of 20 in Wuhan in the 2nd week of January.

As of 20 February, in Guangdong Province, among 9939 identified close contacts, 9939 (100%) were traced and 7765 (78%) have completed medical observation.  Among the close contacts, 479 (4.8%) were found to be infected with COVID-19.

I would think that percentage is dependent upon how quickly the primary patient was diagnosed after becoming infectious.

The report does confirm what I had suspected: all patients (including mild symptoms) who tested positive were admitted to the hospital.  They had no visitors.  They had to test negative twice, 24 hours apart, before being released.   No home care (with other members of the household wandering in & out) that Santa Clara County is currently doing. 

COVID-19 is transmitted via droplets and fomites during close unprotected contact between an infector and infectee. Airborne spread has not been reported for COVID-19 and it is not believed to be a major driver of transmission based on available evidence...[etc]

[M T]

When the virus started to spread in Guangdong — the province where the 2002-2003 SARS outbreak began — worried people flooded fever clinics to be tested. Of 320,000 tests performed, just under 0.5% were positive for the virus at the peak of transmission there, he said — which suggests that only 1 case out of 200 was being missed.

That doesn't make sense or at least the phrasing is misleading.  If the 320,000 people tested were representative and .5% were positive, it doesn't mean that 1 in 200 cases are missed, it means that 1 in 200 people have an undetected case.  The population of Guandong is 113.46 million, so if .5% are positive, that's 56500 undetected cases.  Since there are 1350 cases in the province, that means that >97% of cases are undetected.

Good catch.  I took that quote from the article, not from the WHO-China Joint Mission report.  I shouldn't have trusted it.  The numbers are wrong and the interpretation in the article is faulty, as you pointed out.
From the report

Within the fever clinics in Guangdong, the percentage of samples that tested positive for the COVID-19 virus has decreased over time from a peak of 0.47% positive on 30 January to 0.02% on 16 February.  Overall in Guangdong, 0.14% of approximately 320,000 fever clinic screenings were positive for COVID-19.

At a "fever clinic", the patients will not be asymptomatic and will not be a random sample of the population.  0.14% translates to about 448 patients detected at the fever clinics. 

Interestingly, tests for COVID-19 (mostly in January) of samples of Influenza-like Illnesses had very few positive hits. The highest reported was 3 out of 20 in Wuhan in the 2nd week of January.

As of 20 February, in Guangdong Province, among 9939 identified close contacts, 9939 (100%) were traced and 7765 (78%) have completed medical observation.  Among the close contacts, 479 (4.8%) were found to be infected with COVID-19.

I would think that percentage is dependent upon how quickly the primary patient was diagnosed after becoming infectious.

The report does confirm what I had suspected: all patients (including mild symptoms) who tested positive were admitted to the hospital.  They had no visitors.  They had to test negative twice, 24 hours apart, before being released.   No home care (with other members of the household wandering in & out) that Santa Clara County is currently doing. 

COVID-19 is transmitted via droplets and fomites during close unprotected contact between an infector and infectee. Airborne spread has not been reported for COVID-19 and it is not believed to be a major driver of transmission based on available evidence...[etc]