It's obvious if you know the solution ahead of time: we must conclude that the whole population needs to be vaccinated, and therefore we can work backward to fi

It's obvious if you know the solution ahead of time: we must conclude that the whole population needs to be vaccinated, and therefore we can work backward to find the false positive rate. Now, you can get upset at this explanation, but I challenge you to find a better explanation for starting with the exact, but arbitrarily picked false positive rate that leads to a politically desirable outcome.