WASHINGTON — During a combative three-hour meeting, confrontations arose between U.S. senators and Robert Kennedy Jr. on Thursday, as the nation’s health secretary defended against allegations regarding extensive changes to vaccine policies, healthcare strategies, and leadership shifts he has implemented.

The Senate Finance Committee’s oversight session provided an opportunity for senators to obtain explanations from Kennedy over recent significant exits at the Centers for Disease Control and Prevention, the appointment of vaccine critics to a key federal advisory board, and changes to COVID-19 vaccine guidelines, which could make vaccine access more challenging for many Americans.

Members from both political parties aggressively questioned him about whether he would uphold his previous promise not to prevent access to vaccines for Americans who desired them. Kennedy, visibly frustrated, rejected these claims and cast doubt on the safety and efficacy of vaccines from his influential Capitol Hill platform.

Here are some key takeaways from the hearing:

Kennedy repeatedly contradicted the narrative of ousted CDC director Susan Monarez, dismissed unceremoniously last week after a tenure of less than a month. Monarez, nominated by President Donald Trump, had been recommended for the role by Kennedy and confirmed following a Senate vote in July.

In a Wall Street Journal op-ed released Thursday, Monarez reaffirmed that she was instructed to “preapprove” recommendations by Kennedy’s selected vaccine advisers, many of whom have previously questioned fundamental aspects of vaccine science.

“I asked her: ‘Are you a trustworthy person?’ and she said ‘No,’” Kennedy stated, explaining his change of opinion on Monarez. “If you had an employee who told you they weren’t trustworthy, would you ask them to resign?”

An attorney representing Monarez called Kennedy’s statements “false” and “patently ridiculous,” in a written statement. Her attorney said she would repeat the allegations in her Wall Street Journal op-ed under oath.

Later in the hearing, Kennedy acknowledged that he’d told Monarez to fire several senior CDC officials. Throughout the hearing, Kennedy accused the agency’s scientists of failing to combat chronic disease and making unsound recommendations during the pandemic.

“The people who at CDC who oversaw that process, who put masks on our children, who closed our schools, are the people who will be leaving,” Kennedy said.

For Kennedy, tough questions from angry senators were an opportunity to fight back.

Throughout the hearing, a resolute Kennedy accused senators of lying, misrepresenting his agency and making little sense. As a result, questioning from Democratic senators repeatedly turned into yelling matches.

“You’re just making stuff up,” Kennedy told Sen. Tina Smith, a Democrat from Minnesota, after she accused him of “blaming school shootings on antidepressants.” The health secretary said on Fox News after the recent Minnesota shooting that antidepressants could potentially contribute to violence, and said his agency was investigating. While critics of antidepressants have long alleged they may increase homicidal behavior, the link is not supported by multiple, large clinical trials of the drugs.

When Democratic Sen. Raphael Warnock of Georgia questioned Kennedy about his disparaging rhetoric about CDC employees before a recent deadly shooting at the agency, Kennedy retorted: “Are you complicit in the assassination attempts on President Trump?”

Kennedy said Sen. Ben Ray Luján of New Mexico was “talking gibberish” when the Democrat asked him about the details of his agency’s autism research.

“Mr. Secretary, let me speak slowly and clearly so that you can understand me through my New Mexico accent,” Luján responded.

Democratic senators pressed Kennedy on recent changes narrowing the approval of annual COVID-19 shots. He repeatedly disputed or denied their accusations.

Last month, the Food and Drug Administration approved updated shots but only for seniors or younger people with underlying health risks. That’s sparked confusion and frustration from many Americans, including parents interested in vaccinating healthy children against the virus.

“Why have you acted behind closed doors to overrule scientists and limit the freedom of parents to choose the COVID vaccine for their children?” Democratic Sen. Maggie Hassan of New Hampshire asked.

“This is crazy talk,” Kennedy responded. Later, he acknowledged that access to vaccines at pharmacies “depends on the state.”

In many states, pharmacists are legally barred from administering vaccines outside the uses endorsed by the CDC’s advisory panel, prompting CVS and other pharmacy chains to turn away people seeking shots in certain states.

“You promised that you would not take away vaccines from anyone who wanted them,” said Democratic Sen. Elizabeth Warren of Massachusetts.

“I’m not taking them away from people,” Kennedy said, noting that healthy Americans who want a shot should still be able to get one if a doctor prescribes one. Typically, Americans do not need to see a doctor for annual vaccine updates.

Until this year, the FDA and CDC had recommended yearly COVID vaccinations for everyone ages 6 months and up.

Kennedy could not be pinned down on basic facts and statistics, particularly when it came to vaccines and COVID-19.

In an exchange with Sen. Mark Warner, Kennedy claimed nobody knows how many Americans have died from COVID-19 because of a lack of government data.

Both the CDC and the World Health Organization have concluded that approximately 1.2 million Americans have died from the virus.

“The secretary of Health and Human Services doesn’t know how many Americans died from COVID,” said Warner, a Virginia Democrat. “How can you be that ignorant?”

Republicans also pressed Kennedy on his assessment of Operation Warp Speed, the Trump-led initiative that rapidly developed COVID vaccines in the first year of the pandemic. Trump has long claimed success for the effort, while acknowledging unsubstantiated theories that mass vaccinations may have caused more harm than good.

Kennedy has said it’s unclear how many lives were saved by the vaccines because of imperfect data collected by the CDC and vaccine makers.

Nevertheless, when questioned by Republican Sen. Bill Cassidy of Louisiana — a physician whose vote was key in Kennedy’s nomination — about Operation Warp Speed, Kennedy agreed that Trump “absolutely” deserved a Nobel Prize.

Senate Republicans were more aggressive with Kennedy than they have been with most of Trump’s top officials, with several asking pointed questions about his efforts to limit access to vaccines.

Wyoming Sen. John Barrasso, also a doctor and the No. 2 Senate Republican, said he had grown “deeply concerned” that some vaccines could be in jeopardy after Kennedy had cut research funding and fired the CDC director.

“Americans don’t know who to rely on,” Barrasso said.

North Carolina Sen. Thom Tillis read off a list of questions for Kennedy that he said he wanted answered after the hearing, including where the health secretary stands on the COVID-19 vaccine.

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