When did I say I want to stop millions of people from "traveling"? First off, you can't have a country without borders. Secondly, you can have migration but in a modern world you can't just let people flow through. That's why we had Ellis Island, people could be vetted for things such as deadly diseases and possibly be vaccinated before being let into the country.
We have immunization requirements for foreign countries.
I'd be more worried about states like Missouri and Indiana with nearly a sixth of the population who are not vaccinated against measles.
Mexico, as a counter example, has a nearly 99% vaccination rate for measles.
