The researchers point out that all of the counties predicted to have an outbreak that haven't seen one yet are either adjacent to a county that has an outbreak or has a major international airport in it. That means all of the remaining top 25 counties are at serious risk of developing a measles problem. In the paper, the researchers suggest that these places should be targeted with more vaccination efforts and enhanced measles surveillance. Cook, Illinois and Los Angeles, California, in particular, have each only had one case, but have airports that could, as the researchers put it "serve as the fulcrum of continuous import of the measles virus into the USA." You can see the full list of results here.