The White House said late Wednesday that it had fired Susan Monarez, the new director of the Centers for Disease Control and Prevention, after a tense confrontation in which Health Secretary Robert F. Kennedy Jr. tried to remove her from her position. A lawyer for Dr. Monarez said in response that she was refusing to step down. Dr. Monarez, an infectious disease researcher, was sworn in just a month ago by Mr. Kennedy, but had clashed with the secretary over vaccine policy, people familiar with the events said. Four other high-profile C.D.C. officials quit en masse, apparently in frustration over...