The proposal is to separate people into two groups: those who agree to donate their organs, and those who decline. Later on, when people happen to need organ transplants, they receive them according to their group priority, i.e. if you decline to donate, you turn comes up only after everyone in the first group received their organs.
I think that is a fair system, and could be applied in this case also. If you choose to vaccinate, you are covered, and if you do not, you pay out of pocket.