it breaks my heart to see so many "adults" making "informed decisions" about NOT vaccinating their children. They should really go live in undeveloped countries where vaccines are scarce and unavailable to experience first hand what lack of vaccinations actually means, what the implications are for the general population. I grew up, fortunately with out any problems, seeing so many kids suffering from polio, handicap…
Defecting on vaccination is perfectly rational if you expect everyone else to cooperate and never meet another defector. You get all the benefits of herd immunity while taking on none of whatever slight risk of complications there may be. It's pretty straight-forward game theory, which must be met with equally straight-forward strategies: defectors must be punished.
Herd immunity doesn't apply because the defectors have a closely related sub-herd through which the disease spreads quickly. (On the other hand, it does apply when the disease tries to pass out of the community and cannot. Hm.)