To achieve herd immunity - when enough of a group is immune to a disease, limiting its spread and protecting the unvaccinated - around 95% of the population must have the shots, said Alefiyah Malbari, the chief of ambulatory pediatrics at University of Texas Dell Medical School in Austin.

Texas authorities on Wednesday announced the first measles death in the state's outbreak of the highly contagious disease. The death, of a school-age child, is the first measles death in the U.S. since 2015,