Researchers at the University of Texas at Austin and Johns Hopkins University identified the 25 American counties most at risk of measles outbreaks because of their higher vaccine-exemption rates and proximity to airports. The researchers focused on counties with international airports, because every American outbreak since 2000 has begun with a case imported from overseas. The study, published in “The Lancet Infectious Diseases,” looked at current vaccination rates in counties and what would happen if they dropped.