[amelius]

> The “prank” danger: someone just pressing “I’m infected” for fun.

How about you can only check the "I'm infected" box if your phone is near the phone of a health worker?

Another idea is that you get a code in the mail from the hospital which you have to enter in the app.

[jjgreen]

In the UK, most supermarkets have an hour where only health workers can enter (quite right, give them priority). But this makes it trivial to get close enough to health-worker to execute your prank, just join that queue.