You're absolutely right that the word 'rational' is so overloaded that it can be nearly meaningless -- that's why I qualified it with 'selfish'. We do need to

You're absolutely right that the word 'rational' is so overloaded that it can be nearly meaningless -- that's why I qualified it with 'selfish'. We do need to draw distinctions between consequences that are immediately faced by the decisionmaker, vs. the long-term consequences to the group. Note that even if you do care about the long-term consequences to the group, it may still be narrowly rational to decide not to immunize. After all, your own individual decision is only going to have a tiny impact on whether herd immunity persists in your community, but will have a much greater impact on yourself. This is the same logic over why companies pollute, people litter, fishermen overfish -- there is a 'commons problem' where people collectively benefit from collaboration, but selfish individuals can benefit by defecting and free-riding. If one can't see how selfish behavior may appear rational, then it becomes very difficult to figure out how to structure norms and incentives to encourage group cooperation.