On: Citing parental freedom, Arizona lawmakers move ahead with anti-vaccine bills

This doesn't explain the more severe measles epidemics in low-Internet-usage countries like Vietnam.

Measles doesn't spontaneously appear within a community; it's brought in from the outside.

I have not looked into the Vietnam case but here's an report about how Costa Rica had not had a single case for 5 years until an unvaccinated french child presented at a hospital with the disease a few days ago. https://boingboing.net/2019/02/25/tourism-brings-the-measles...

There might well be a different explanation for outbreaks in different places; I am certainly not asserting this as a universal causal factor.