A doctor explained to me that it has to do with herd immunity. Many vaccines are not 100% effective. Some are only 90% effective or worse. If a vaccine only takes effect in 90% of the people who take it, that is a large enough quantity to act as an insulator to help prevent the disease from spreading to those who the vaccines fails.

In this way, it really makes no difference at all if a small percentage of people want to be anti-vaxers - the problem arises when the percentage becomes very large and it begins to allow certain diseases to "break through" the herd immunity.

It's said "the level of vaccination needed to achieve herd immunity varies by disease but ranges from 83 to 94 percent"