US Health Secretary Robert F Kennedy Jr defended his decisions to fire scientists and overhaul the nation's vaccine policies on Thursday as he came under blistering fire from Democrats urging him to resign during a Senate hearing.

The three-hour grilling, which often erupted into shouting matches, came a week after the Trump administration's ousting of Sue Monarez, the director of the Centers for Disease Control and Prevention (CDC), which plunged the nation's premier public health agency into turmoil.

In his opening remarks, Kennedy tore into the CDC's actions during the Covid pandemic, accusing it of failing "miserably" with "disastrous and nonsensical" policies including masking guidance, social distancing and school closures.

"We need bold, competent and creative new leadership at CDC, people able and willing to chart a new course," he said, touting the health department's new focus on chronic disease.

Monarez, the CDC director whom Kennedy previously endorsed, accused the secretary of a "deliberate effort to weaken America's public-health system and vaccine protections" in a Wall Street Journal op-ed on Thursday.

Kennedy's explanation for her firing – as he told senators during the hearing – was simply: "I asked her, 'Are you a trustworthy person?' And she said, 'No.'"

"Secretary Kennedy's claims are false, and at times, patently ridiculous," Monarez's lawyers said in a statement sent to AFP, adding she would be willing to testify under oath.

Once a respected environmental lawyer, Kennedy emerged in the mid-2000s as a leading anti-vaccine activist, spending two decades spreading voluminous misinformation before being tapped by President Donald Trump as health secretary.

Since taking office, Kennedy has restricted who can receive Covid-19 shots, cut off federal research grants for the mRNA technology credited with saving millions of lives, and announced new research on debunked claims linking to autism.

Ron Wyden, the top Democrat on the Senate Finance Committee leading the hearing, set the tone by demanding Kennedy be sworn in under oath – accusing him of lying in prior testimony when he pledged not to limit vaccine access.

"It is in the country's best interest that Robert Kennedy step down, and if he doesn't, Donald Trump should fire him before more people are hurt," Wyden thundered.

But Republican committee chairman Mike Crapo shot down the request.

The exchanges only grew more ill-tempered.

Democratic Senator Maria Cantwell branded Kennedy a "charlatan" over his attacks on mRNA research, while Kennedy accused Senator Maggie Hassan of "crazy talk" and "making things up to scare people" when she said parents were already struggling to get Covid vaccines for their children.

Vaccines have become a flashpoint in an ever-deepening partisan battle.

Conservative-leaning Florida on Wednesday announced it would end all immunisation requirements, including at schools, while a West Coast alliance of California, Washington and Oregon announced they would make their own vaccine recommendation body to counter Kennedy's influence at the national level.

But RTHK's US correspondent, Mark Niu, said that vaccination rates among kindergarten children have been dropping steadily across the US, even before the recent developments.

"[Florida] currently requires children in public schools and child care facilities to receive shots for chicken pox, measles and polio, among other diseases. But the state - like many others - offers religious and medical exemptions from its vaccine mandates," he told Hong Kong Today.

"Experts I spoke to, even in the state of California, for example, say that exemptions are more often requested from parents today for their children, and more often they are getting approved, in part due to the political climate."

Republicans mostly closed ranks around Kennedy, though there was some notable dissent.

Senator Bill Cassidy, a physician whose support was key to Kennedy's confirmation, criticised his cancellation of mRNA grants. He was joined by fellow Republican doctor Senator John Barrasso and Senator Thom Tillis.

Cassidy pressed Kennedy on whether President Trump deserved a Nobel Prize for Operation Warp Speed, the program that sped Covid vaccines to market.

Kennedy agreed Trump should have received the prize – but in nearly the same breath, praised hydroxychloroquine and ivermectin, drugs championed by conspiracy theorists that have been proven ineffective against Covid-19. (AFP/RTHK)