OK, so only the unvaccinated get the disease. What’s the issue?
It’s possible vaccinated can get it. Nothing is perfect.
Because infants that are too young to vaccinated get it. Even if you get your children vaccinated on schedule, your child can still get it if they come in contact before they are old enough to be vaccinated.
Because some of the in mmunized can’t. Too young, or immunocompromised.
When the total number of unimmunized drops below a critical number you lose the herd immunity effect, and you can get person to person spread and epidemics.
It’s more than that. It’s called herd immunity.
For various reasons, some still not understood to science, some people do not become immune after vaccination. They can get very severe, potentially life-threatening disease if they are exposed. Also, children too young to be vaccinated are at risk, as are those who have immune system disorders.