>doctor gives you a one-time code Realistically implementing this kind of system in all countries affected by COVID will take more time (arguably) then vaccine development. It’s way more effective (arguably) to align financial incentives such that users are interested in providing accurate data via (self) reporting and have no incentives conversely in proving false data.

As for fingerprinting - entirely untrue, eth addresses are completely pseudonymous, in stark comparison to App Store/Play Store apps which are tied to your phone number/CC info et al.

What's to prevent anyone from saying they have Covid without a valid test?

You'll need to have verified doctors / tests to generate codes for uploading.