WASHINGTON, Aug. 11 -- The president signed the order in the afternoon. By evening, the country's leading pediatric organization had called it dangerous. By morning, the people responsible for administering vaccines to children had been given, by the executive branch, a competing set of instructions.

President Trump signed an executive order Monday restructuring the federal childhood vaccine schedule, a framework that had been largely stable for decades, directing that the combination measles, mumps and rubella shot be broken into three separate injections administered at three separate appointments. The order would reduce universal vaccine recommendations from 18 diseases to 11, shifting vaccines against meningococcal disease and hepati...