I believe reward system for covid free wouldn't work really well considering the testing part of it. However rewarding people who do stuff that should implicate less chance for covid infection would be better. Like for example you could have people download apps that show where they are and they are always at home, not going outside, this should be getting rewards, or I am biased because I haven't left my home in 50+ days to prevent from infection
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You could find things like that, which would be things that prevent easy covid sharing like going outside for a restaurant or movie or whatever, since those are risky, so reward those who do not do those things. Or simply yet, just reward ones that get vaccination, those are surely people who would be less possible to die from covid, and that is why I think it would make a lot more sense to reward those.