> If everyone else is immunized to the point where there is significant herd immunity, but immunization has a low but non-zero cost (e.g. feeling crappy for a few days), then the rational selfish action is to be a free rider and not immunize.

Except that's not really how it is. There's still a very non-negligible risk to catch any of a number of vaccine-preventable diseases. People die from measles on a regular basis.

You can in theory imagine a situation where this is different, but realistically Vaccines totally make sense even from a very egoistic perspective.