Secretary Robert F. Kennedy Jr. faced intense questioning from Republican and Democratic senators alike at a combative hearing on Thursday. The lawmakers, who grilled the Department of Health and Human Services secretary about his firing of the Centers for Disease Control director, focused much of their concern on an advisory panel meeting later this month that could lead to some vaccines being taken off the market. 

Mr. Kennedy has been under fire since last week’s dismissal of the CDC director, Susan Monarez, whom the secretary himself had praised when she was sworn in weeks earlier. In justifying the firing, the secretary told the senators that Ms. Monarez had replied, “No,” when he asked her if she was a trustworthy person.

Ms. Monarez, in a Wall Street Journal opinion piece published Thursday, told a different story. She wrote that Mr. Kennedy had demanded she “preapprove the recommendations of a vaccine advisory panel newly filled with people who have publicly expressed antivaccine rhetoric.” 

That panel, the Advisory Committee for Immunization Practices, is set to meet on September 18 to consider new recommendations for this year’s Covid booster shots, as well as the Hepatitis B vaccine; the MMRV vaccine protecting against measles, mumps, rubella, and varicella, and the Respiratory Syncytial Virus vaccine.

Some of the most pointed questioning Thursday came from Senator Bill Cassidy, the chairman of the Health, Education Labor, and Pensions Committee; he is a physician, though Thursday’s hearing was before the Senate Finance Committee.

Dr. Cassidy last week called on the ACIP to delay its September meeting amid complaints of a lack of scientific rigor in the vaccine recommendation process. Otherwise, Dr. Cassidy said, any recommendations from the panel should be “rejected.”

At the hearing, Dr. Cassidy spoke immediately after Senator Michael Bennet, a Democrat who had engaged in a shouting match with Mr. Kennedy about vaccines and their safety. 

“Do you agree with me that the president deserves a Nobel Prize for Operation Warp Speed?” Dr. Cassidy asked, referring to the federal initiative that helped the private sector develop a Covid vaccine in just 10 months. 

“Absolutely,” Mr. Kennedy responded. 

“But you just told Senator Bennet that the Covid vaccine killed more people than Covid,” Dr. Cassidy said. “That was a statement that you just made.”

“I did not say that,” Mr. Kennedy shot back. Although Mr. Kennedy did not specifically say that the Covid vaccine killed more people than Covid, he did say at the hearing that mRNA vaccines like the ones developed during Operation Warp Speed caused death in some instances.

Dr. Cassidy, who had overseen Mr. Kennedy’s confirmation process in his own committee, then moved on. He said he was “surprised” that Mr. Kennedy would praise Operation Warp Speed even though he has been cutting contracts for mRNA vaccine research. 

“It seems inconsistent that you would agree with me that the president deserves a tremendous amount of credit for this,” Dr. Cassidy said. 

“Is this a question Senator Cassidy, or is this a speech that you don’t want me to answer?” the health secretary asked in response. 

Dr. Cassidy also criticized Mr. Kennedy’s decision to fire the 17 members of the ACIP board and replace them with his hand-picked appointees, many of whom have long expressed skepticism about vaccinations. The Louisiana senator noted that Mr. Kennedy constantly derides those with “conflicts of interest” in the pharmaceutical and healthcare industries, but seems to have ignored alleged conflicts faced by his ACIP appointees. 

“Many of those whom you have nominated for ACIP have received revenue [from] serving as expert witnesses for plaintiffs’ attorneys suing vaccine makers,” Dr. Cassidy stated. “If we put people who are paid witnesses for people suing vaccine [manufacturers], that actually seems like a conflict of interest. Real quick: Do you agree with that?”

“No, I don’t,” Mr. Kennedy responded curtly. 

At the end of his remarks, Dr. Cassidy submitted to the record an email message he received from a friend who is also a doctor. That friend asked the senator whether he might face legal liabilities for giving out Covid boosters in light of Mr. Kennedy’s public statements and what Dr. Cassidy called “conflicting” messaging and guidelines coming from Mr. Kennedy’s department. 

“I would say effectively we’re denying people vaccines,” Dr. Cassidy said. 

“You’re wrong,” Mr. Kennedy shot back. 

Senator John Barrasso, a trained orthopedic surgeon, brought up Mr. Kennedy’s promise to uphold the highest standards for making sure vaccines are safe and for ensuring that they are accessible. Dr. Barrasso serves as the Senate majority whip, making him the second-ranking Republican in the chamber behind majority leader. 

“I believe one of President Trump’s greatest achievements was his bold and successful actions on Covid,” the senator said in his opening remarks, calling Operation Warp Speed a “model of American ingenuity.”

“I support vaccines. I’m a doctor. Vaccines work,” Dr. Barrasso said. 

“I’ve grown deeply concerned,” he continued. “The public has seen measles outbreaks, leadership at the National Institutes of Health questioning the use of mRNA vaccines, the recently confirmed director of Centers for Disease Control and Prevention fired,” Dr. Barrasso told the secretary. “Americans don’t know who to rely on.”

“If we’re going to make America healthy again, we can’t allow public health to be undermined,” the senator added. 

Dr. Barrasso said that he, like Dr. Cassidy, had been hearing from old friends from his days in medical school, in his residency, and in his private practice who are deeply concerned that safe and effective vaccines could be taken off the market by the ACIP. 

“That would put Americans at risk and reverse decades of progress,” Dr. Barrasso said.