The problem is that as effective as vaccines are, they still have a failure rate. A certain number of VACCINATED individuals are still going to get break-through infections. Usually the incidents of a disease are so rare it never affects the vaccinated, but in a break-out like this, a lot more vulnerable vaxxed individuals have a lot more contact with sick people, and they catch it. That gives the disease the opportunity to mutate around the known vaccines, rendering the current vaccines useless.
They aren't just threatening other anti-vaxx dummies, they are affecting ALL of us.