https://www.nytimes.com/2020/08/29/h...s-testing.html
This is interesting.
The PCR tests can be extremely sensitive at detecting COVID virus. So sensitive they can test sewage to see if there are infected people in a city.
The PCR amplifies the viral RNA with each cycle. You can run as many cycles as you want in order to crank up the sensitivity. The fewer cycles required to turn positive, the more virus on the swab. The more cycles required to turn positive, the less virus on the swab.
Looks like no one in USA has agreed how many cycles constituted a real positive (who may get sick and may infect others) from someone with minuscule traces of virus that pose zero risk.
Most experts suggest the test should be limited to 30 cycles.
But most tests do 40 cycles.
If you stopped after 30 cycles and called the test negative, 90 % of current positive results would be negative. So USA would have 4000 meaningful infections each day rather than 45,000 cases.
This helps explain all the asymptomatic cases - the test is repeated until it turns positive, even though most of these positives won’t get sick.
It also means we have been quarantining and isolating many people who are not going to get sick and who won’t infect others.
If China are still testing, maybe they have set a much lower cycle threshold. And thats why numbers are way down there.