Yeah, those US long-time residents who didn't get their kids vaccinated. Those fall into two groups: Those opposed to vaccinations and those who simply didn't because of cost or accessibility or awareness.
Don't know "what's out there" these days, but when I was much younger and in a different state those opposed to vaccinations fell into three groups: those opposed for religious reasons and usually on the right or other (quasi)ideological, often "health" related, reasons and often on the left. Sort of an argument from bodily autonomy--"I control my body and am responsible for my kid's bodies, and the state will not tell me what I must do". (We can argue that there's a social responsibility to be imposed on others, right? While I like my own values on the matter and dislike other's values, values are personal but compulsion is societal.)
Then there's the other large reason actually cited in the OP:
the largest of which took off at a migrant shelter in Chicago in March and was linked to more than 60 cases
Note Chicago isn't a deep red bit of the US. And while this year's figures are 3x last year's, that one outbreak accounts for pretty much 1/3 of that or half the increase. Dispose of that one event and we're "just" double last year's figures.
A lot of countries have had vaccination campaigns and some report very high vaccination rates--but they also report very high rates of corruption. And in some countries they have Muslim fundies and CTers that are convinced that vaccinations are a Western anti-Muslim sterilization attempt. Where you find Muslim insurgencies you find such fairly often and it's not surprising that strains of polio that surface in Nigeria, Mali, Pakistan and Afghanistan are all genetically the same. This means that some immigrants, Muslim or otherwise--the percentage is not likely to be incredibly high over all, but ultimately it's an empirical question I haven't seen answered--will not be vaccinated. Why does this matter for the current US measle incidence?
declining vaccination rates in the U.S. and a rise in measles cases worldwide.
More measles in other countries + travel between countries (from there to here or just round trip) translates into greater incidence here.
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