A crazy amount of statistical reasoning would have to be put into such an app to not trigger a false positive after a long shopping tour for instance and still

A crazy amount of statistical reasoning would have to be put into such an app to not trigger a false positive after a long shopping tour for instance and still have a reasonable true positive rate. The current numbers publicly available aren't even exploited to their fullest. IMHO assuming that the lethality is a constant (or a polynomial) would already be a treasure trove of knowledge, in order to approximate the number of untested cases. Also looking at the WHO dashboard, I have serious doubts anyone cares if there is actionable value for the population available. Maybe it would be more useful to start centralizing the numbers available already (PPE supplies, hospital usage, ...), throwing them into more realistic simulations and testing models. IMHO it's still not really clear why China has less cases than the US and why Taiwan and Sweden are doing just fine with only conservative lockdown measures. FWIW, it would also be helpful if Chinese institutions would publish if their tracking efforts have brought actually a provable difference.