U.S. Health Secretary Robert F. Kennedy Jr., facing pointed bipartisan questioning at a rancorous three-hour Senate committee hearing on Thursday, tried to defend his efforts to pull back COVID-19 vaccine recommendations and explain the turmoil he has created at federal health agencies.

Kennedy said the fired CDC director was untrustworthy, stood by his past anti-vaccine rhetoric, and disputed reports of people saying they have had difficulty getting COVID-19 shots.

A longtime leader in the anti-vaccine movement, Kennedy has made sweeping changes to agencies tasked with public health policy and scientific research by laying off thousands of workers, firing science advisers and remaking vaccine guidelines. The moves — some of which contradict assurances he made during his confirmation hearings — have rattled medical groups and officials in several Democratic-led states, which have responded with their own vaccine advice.

Medical groups and several Democrats in Congress have called for Kennedy to be fired, and his exchanges with Democratic senators on the panel repeatedly devolved into shouting, from both sides.

But some Republican senators also expressed unease with his changes to COVID-19 policies.

The GOP senators noted that Kennedy said President Donald Trump deserved a Nobel Prize for the 2020 Operation Warp Speed initiative to quickly develop mRNA COVID-19 vaccines — and that he also had attacked the safety and continued use of those very shots.

Other Senators asked him why the director of the Centers for Disease Control and Prevention was fired last week, less than a month into her tenure.

Kennedy said she was dishonest, and that CDC leaders who left the agency last week in support of her deserved to be fired.