The White House on Thursday said President Donald Trump has fired Centers for Disease Control and Prevention Director Susan Monarez after she refused to resign.

Hours later, NBC News reported the White House had tapped Jim O’Neill, deputy secretary of the Health and Human Services department, to serve as acting director of the CDC. O’Neill was sworn in as deputy secretary in June, and is a key aide to HHS Secretary Robert F. Kennedy Jr.

The temporary appointment suggests that Kennedy could have a clearer path to make changes to U.S. immunization policy, particularly after Monarez had refused some of his requests. The permanent replacement for Monarez will have to be confirmed by the Senate.

During a briefing on Thursday, White House press secretary Karoline Leavitt said Trump fired Monarez, “which he has every right to do.” She said Trump has “the authority to fire those who are not aligned with his mission,” and that he or Kennedy Jr. will announce a new CDC director “very soon.”

Earlier Thursday, Monarez’s attorney Mark Zaid said Monarez would remain in the role because she is a presidential appointee and only Trump can fire her. Zaid said White House personnel had tried to fire her, not the president.

In a statement, lawyers for Monarez said they were “not aware of anything new happening.”

“Receiving an email from an HR staffer simply saying ‘you’re fired’ is insufficient as a matter of law to constitute the termination of a federal employee, especially one appointed by the president and confirmed by the Senate,” Zaid said.

He also said she “refused to rubber-stamp unscientific, reckless directives and fire dedicated health experts” and that “she chose protecting the public over serving a political agenda.”

“For that, she has been targeted,” he said.

Monarez and Kennedy were at odds over vaccine policy, The New York Times reported Wednesday, citing an anonymous administration official.

Kennedy, a prominent vaccine skeptic, has taken several steps to change immunization policy in the U.S.

Monarez was sworn in on July 31. A longtime federal government scientist, she is the first CDC director to be confirmed by the Senate following a new law passed during the pandemic that required lawmakers to approve nominees for the role.

Trump’s move to oust her is the latest in a leadership upheaval at the CDC.

At least four other top health officials announced Wednesday that they were quitting the agency shortly after HHS said Monarez was “no longer” the director of the CDC in a post on X.

In a Fox News interview Thursday morning, Kennedy declined to comment on “personnel issues.” But he said the agency “is in trouble, and we need to fix it, and we are fixing it, and it may be that some people should not be working there anymore.”

Kennedy said Trump has “very, very ambitious hopes for the CDC right now.” But he said the CDC “has problems,” claiming that the agency took the “wrong” approach when it came to social distancing, masking and school closures during the Covid pandemic.

“We need to look at the priorities of the agency, if there’s really a deeply, deeply embedded … malaise at the agency, and we need strong leadership that will go in there and that will be able to execute on President Trump’s broad ambitions for this agency, the gold standard science and to what it was when we were growing up, which was the most respected health agency in the world,” Kennedy said.

The leadership departures come at a tumultuous time for the agency, which is reeling from a gunman’s attack on the CDC’s Atlanta headquarters on Aug. 8. A police officer died in the shooting. 

Correction: This article has been updated to reflect the correct day the White House said Trump fired Susan Monarez after she refused to resign, and to reflect the correct wording of Robert F. Kennedy Jr.’s last quote.

— CNBC’s Angelica Peebles contributed to this report.