Kennedy fired Dr Monarez less than a month after she was sworn in as head of the agency that leads the US response to infectious and chronic diseases, adding to a heated political fight over the changes he has made to his department this year.

Testifying alongside the former chief medical officer for the agency, who resigned after the firing, Dr Monarez told the health committee she had a meeting with Kennedy on 25 August, where he "demanded" two things from her.

The first was that she approve every recommendation from the Advisory Committee on Immunization Practices. Kennedy had dismissed the committee's full membership in June, and has appointed all of its new members

The second, Dr Monarez said, was "to dismiss career officials responsible for vaccine policy, without cause."

When she refused to resign, Kennedy said he had already spoken to the White House about having her removed, she said, adding he informed her he was in frequent communication with President Donald Trump about changes to US vaccine policy.

Kennedy, a notable vaccine sceptic, has previously denied that she was told to sign off on vaccine recommendations that did not have a scientific basis.

Dr Monarez also testified that Kennedy called CDC staff "horrible people" and the health agency "corrupt".

Her dismissal came shortly after after a man allegedly shot 180 rounds at the CDC headquarters, killing the responding police officer, in protest of Covid vaccines. It made headlines around the world.

Dr Monarez then published an opinion article in the Wall Street Journal that "the Senate confirmed me to ensure that unbiased evidence serves our nation's health, and for doing that, I lost my job", while Kennedy told a different congressional hearing that he let her go because she was not trustworthy.

Soon after she was fired, other leaders at the CDC resigned, including then-Chief Medical Officer Debra Houry, who also testified on Wednesday.

CDC head says she was fired for refusing RFK Jr's vaccine changes