A vaccine that is 99% efficient (and most are not 100% efficient!) is probably sufficient to eradicate the disease in vaccinated populations ("herd immunity");

A vaccine that is 99% efficient (and most are not 100% efficient!) is probably sufficient to eradicate the disease in vaccinated populations ("herd immunity"); but if you suddenly enter a population where the disease is prevalent, you have a nearly 1% probability of falling ill, which is much higher than in vaccinated populations. Of course, "no responsible parent" is hyperbole. But there is a significantly higher risk even for immunized children. (Found this on your thread page; I hope this was helpful.)