There was, and is, always the chance of encountering someone unvaccinated, or who can't be for some reason. Particularly those travelling - for whatever reason.

There was, and is, always the chance of encountering someone unvaccinated, or who can't be for some reason. Particularly those travelling - for whatever reason. Transplants and chemo can rob your immune system of protection. That's why you need enough vaccinations in the general population to achieve herd immunity that gives indirect protection to those exceptions too. A case cannot spread reliably or rapidly enough to become a significant outbreak. The amount you need to vaccinate to achieve it varies depending on disease - measles needs quite a high percentage vaccinated, others less.