Measles incidence rate per 100,000 people

Interesting chart.

Lots of things to wonder about. For example, it looks like incidence of measles was waning in the decade before the introduction of the vaccine. Exceptions include Kentucky, Tennessee, Hawaii, and Alaska, all of which -- I imagine -- hit some population/transportation threshold that allowed diseases to spread more easily. But the rest generally have incidences that are decreasing, and certainly lower than they were around 1940. Why?

Also, there are virtually no measles cases in 1945. We get light pink in Utah, Washington, California, and Idaho -- and nowhere else that I can see. Of course, any disease is going to have its off years. But 1945 witnessed the end of WWII, and I would think the resulting movements of large numbers of people would have increased both person-to-person contact and mobility, making it prime time for an epidemic. Apparently not. Why not?