idk what you are defining as high vs low density, but the washington DC area is one of the denser areas of the US, and is also the southern end of the northeast megalopolis, which includes some of the densest areas of the US. Idk and i don't think any of those places can compare to China, especially not DC, but its not like you're out living in farmland or something. Its dense cities surrounded by dense suburbs up and down that part of the Atlantic Coast.

Also the locations of those pneumonia outbreaks are Marshall County, WV, outside Phoenix, AZ and Terre Haute, IN. That's low density - the closest city to Marshall County is like Wheeling. If this thing could've spread to those sorts of places, then there can't be too much of a problem with density, and the virus already has access to the densest areas of the US, being situated on the northeast Atlantic Coast. Which is also a very global population with a lot of money, which should mean it can spread globally since those places travel a lot. At least if it can spread to the middle of West Virginia.

Just trying to be critical before I buy this theory