Vaccines don't always make a person 100% immune to a disease, they provide enough immunity so that a disease does not have enough hosts to provide critical mass to spread to epidemic proportions or to mutate into a more virulent form that the vaccine cannot address.
The people who are resisting vaccines are creating a risk for the rest of society by producing a pool of host with little or no immunity that will allow a serious infectious disease to mutate into a more virulent strain and infect everyone, even those who have been vaccinated.
We have been so successful at eradicating some of these once devastating diseases that people have no experience with a full blown epidemic of a serious or deadly disease.
It is what it is.