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.
if people tend to do something because it's easier for them but that will come the long way round and bite them, and a lot of other people, in the end then that's a demonstration of a failure of human to be rational.