Whenever I see a result of 0% (or 100%) with reference to a biological system I am immediately suspicious.
Here is a quote from the comments below the Nature paper :
""There were no positive tests amongst 1,174 close contacts of asymptomatic cases."
How is this possible? Even if all those contacts were indeed uninfected, where are the false positive tests? Even assuming just a 1% false positive rate, the chance of getting zero positives out of 1,174 is less than 1 in 100,000. In the "source data" section, there's an even more improbable claim: zero positive results out of 85,884 tests conducted in East Lake Scenic Area of Wuhan.
Isn't this clear evidence that the data being used is unreliable?"
The false positive rate for PCR tests in SARS-COV-2 is a subject of some contention ; some virologists maintain that there is a nil rate in the strict sense but do admit that e.g. contamination, clerical error etc can introduce false positives into the data base. I have seen a figure of 0.5% offered.
Now, the Nature article seems to make no mention of "false positives". Are the researchers using different criteria for positive/negative? The obvious one is the number of "cycles" for which the test runs ; <37 was positive, >40 was negative, in between attracted a retest. The cycle figures seem to be the accepted standard. Another factor may be the "target genes", in this case 2 in number. Many tests use just 2 but some use 3.
Even if the percentage of false positives was barely above zero, there should still be a substantial number in almost 10 million tests. Is the testing procedure so good that false positives have essentially been eliminated or is there some other explanation?
My usual ending : Covid-19 : we just don't know...yet