Regarding China, asymptomatic at the time of testing doesn't mean it stays that way. If they're doing a testing blitz and testing basically everyone multiple times while the number of cases is in a fast upswing, they're likely catching tons of pre-symptomatic infections. I believe China also uses a ridiculously high cycle count threshold for detecting cases, so they're more likely to catch cases that we wouldn't even consider infections, as well as ones where the person's immune system largely neutralized the virus (PCRs can't distinguish between viable and killed virions).
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