The U.S. eliminated measles a quarter century ago, but to keep its “measles-free” designation, officials will have to make a strong case that measles is not continuously spreading within its borders. Our analysis shows how difficult that could be.

Ongoing outbreaks in Florida, Pennsylvania, Utah and Virginia threaten the country’s measles elimination status.

The U.S. eliminated measles a quarter century ago, but to keep its “measles-free” designation, officials will have to make a strong case that measles is not continuously spreading…