Tuesday marks one year since a measles outbreak started in West Texas, and there have been new cases in the United States each week since. Texas reported more than 760 cases and the deaths of two children before declaring the outbreak over in August.

But under PAHO’s definition of elimination, borders matter. If, for example, the chain of measles that started in the U.S. spread to Mexico and then returned to the U.S. anew, it would be considered a new chain, experts said. Still, many experts call that standard outdated.