When you see an animal, there’s likely a bunch of the same species you didn’t see. Specially if it’s a small animal, with a fast lifecycle, and the animal burrows itself into something (like, dunno… the flesh of another animal?). And if the animal can live pretty much anywhere there’s another, warm-blooded, animal living. (Livestock? Wild fauna? Pets? Humans? Yes.)
So a dozen cases isn’t just “a dozen cases”, there’s likely millions of those flies in USA already. I’m taking a wild guess here and say a billion dollars won’t even scratch the surface of the problem there.
(Not that it changes things for me. Here in South America the fly in question goes from “present” to “present”. Just businesses as usual.)
In general humans are the least concern. We’re smart enough to know something is wrong with our bodies, and fix it before it gets worse; we wear clothes and bandage wounds so there’s less exposure of vulnerable areas; etc. It’s a bit more concerning because of children, since the flies can attack eyes and mouths, but as long as the parents actually do their job and take care of the kid, no issue. (Bug repellent, pay attention to small wounds, regular visits to the doc, this kind of stuff.)
Dogs and cats are another can of worms (or maggots). Specially urban strays; if anyone here wants some nightmare fuel, websearch images for [NSFL] miíase cachorro or miasis perro [/NSFL], apparently the flies (it isn’t just C. hominivorax) responsible for this sort of infestation will lay multiple eggs in the same wound, if they can; so it can get really nasty. Same deal with the fauna.