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 and she refused to resign.

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 vaccine policy and Mr. Kennedy’s leadership.

Because Dr. Monarez had been confirmed by the Senate — previous C.D.C. directors were not subject to such confirmation — she served at the pleasure of the president. Mr. Kennedy most likely did not have the authority to dismiss her.

Her lawyers insisted she was staying put. But at 9:30 p.m., a spokesman for President Trump, Kush Desai, said in an email message that Dr. Monarez had been terminated.

“As her attorney’s statement makes abundantly clear, Susan Monarez is not aligned with the President’s agenda of Making America Healthy Again,” Mr. Desai wrote.

“Since Susan Monarez refused to resign despite informing HHS leadership of her intent to do so, the White House has terminated Monarez from her position with the C.D.C.”