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Senator Bill Cassidy, a Republican physician and vaccine proponent who is facing a primary challenge from the right, has a fraught relationship with the health secretary.
By Sheryl Gay Stolberg and Megan Mineiro
Sheryl Stolberg, who covers health policy, reported from Washington, and Megan Mineiro, who covers Congress, from the Capitol.
Sept. 16, 2025, 11:21 a.m. ET
Three hours after the health secretary, Robert F. Kennedy Jr., threatened to push her out of her job, Susan Monarez, then the director of the Centers for Disease Control and Prevention, picked up her phone and alerted Senator Bill Cassidy, Republican of Louisiana, that she might not be around much longer.
Mr. Cassidy, the physician and ardent proponent of vaccines who leads the Senate’s health committee, had agonized before casting a crucial vote to pave the way to confirm Mr. Kennedy, the nation’s loudest vaccine critic. At the time, the senator said he had done so based on a promise from the nominee that he would protect access to vaccinations and maintain an “unprecedentedly close, collaborative working relationship” with Mr. Cassidy.
The phone call in August from Dr. Monarez, which was recounted by two people familiar with the private exchange who discussed it on the condition of anonymity, suggested that the health secretary had broken both pledges. She detailed for Mr. Cassidy how Mr. Kennedy had told her to fire top C.D.C. officials and accept the vaccine recommendations of his handpicked advisers or resign.
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Dr. Monarez was fired by the White House two days later. Mr. Kennedy says he pushed her out for refusing to say she was “trustworthy.” Now Mr. Cassidy is in a tricky spot, his job imperiled by a primary challenge at home and his reputation on the line in Washington as he determines how far to go in trying to rein in a health secretary who has repeatedly defied Congress — and him personally.
Mr. Cassidy will be in the spotlight on Wednesday, when Dr. Monarez will appear before his committee at his invitation to tell her story publicly for the first time. It is the latest high-stakes moment for a Republican senator who has inhabited shaky political ground ever since he aroused the wrath of President Trump four years ago by voting to convict the president on an impeachment charge of inciting the Jan. 6, 2021, attack on the Capitol.
John M. Couvillon, a Republican pollster based in Baton Rouge, La., said Mr. Cassidy’s public waffling over and eventual support for Mr. Kennedy has left the senator “in big trouble” in his home state, which Mr. Trump won last year with 60 percent of the vote.
“He dithered before making a decision,” about Mr. Kennedy, Mr. Couvillon said. “You don’t gain points, so to speak, for dithering.”
At the same time, public health leaders, many of whom have long respected Mr. Cassidy, now say they are profoundly disappointed in him.
Dr. Georges Benjamin, the executive director of the American Public Health Association, said Mr. Kennedy’s “dysfunction, in many ways, undermines Cassidy’s credibility.” The senator had tried to “thread a fine line,” he said, “and he got burned.”
Dr. Celine Gounder, an infectious disease specialist and editor at large for public health at KFF Health News, said Mr. Cassidy’s vote “looks increasingly at odds” with his “stated commitment to vaccine access.”
And Dr. Paul Offit, a nemesis of Mr. Kennedy who directs the Vaccine Education Center at Children’s Hospital of Philadelphia, issued a public letter to Mr. Cassidy in May, which he also emailed directly to the senator, calling for the health secretary’s resignation.
“Senator Cassidy,” Dr. Offit wrote, “it is not too late to do something about this.” He said he never got a reply.
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A spokesman for Mr. Cassidy, who has been exceedingly reluctant in recent days to discuss his views on Mr. Kennedy, did not respond to requests for comment and declined to make him available for an interview. Mr. Kennedy’s spokesman declined to answer questions about his relationship with the senator.
But the two have tangled for months, and earlier this month, Mr. Cassidy appeared to suggest that Mr. Kennedy’s moves since taking office amounted to a violation of his pledge to senators.
“Effectively, we’re denying people vaccine,” Mr. Cassidy told him at a contentious Senate Finance Committee hearing, drawing an angry response from Mr. Kennedy, who shot back, “You’re wrong.”
It was the most the senator had said publicly since Dr. Monarez’s firing, when Mr. Cassidy said that her departure and the resignations of other high-profile C.D.C. leaders including Dr. Debra Houry, the former chief medical officer, “require oversight” by his committee. Dr. Houry is also set to testify on Wednesday.
Mr. Cassidy’s colleagues said he is focused less on his political survival than on doing his job as chairman of the panel charged with scrutinizing the health department.
“Cassidy is a courageous person who, at the end of the day, concerns himself more about policy than politics,” said Senator Thom Tillis, the North Carolina Republican who announced over the summer that he would not seek re-election after his own tangle with Mr. Trump. “It’s oversight. It’s not personal.”
Mr. Cassidy “does what he thinks is right,” said Senator Susan Collins, Republican of Maine, who like Mr. Cassidy is up for re-election in 2026 and has expressed concern about Dr. Monarez’s firing. She declined to speak about whether Mr. Cassidy’s oversight work might cost him his job.
Mr. Cassidy was initially careful about challenging Mr. Kennedy. Back in February, he declined to criticize the secretary for giving a weak endorsement of the measles vaccine amid an outbreak in West Texas. He instead said that what was important was “the gestalt” of Mr. Kennedy’s remarks, which Mr. Cassidy characterized as: “Let’s get vaccinated!”
Then in June, when Mr. Kennedy fired all 17 members of the C.D.C.’s Advisory Committee on Immunization Practices, or A.C.I.P., and appointed replacements, some of whom share his suspicion of vaccines, Mr. Cassidy replied with something short of a critique on social media.
“Of course, now the fear is that the ACIP will be filled up with people who know nothing about vaccines except suspicion,” Mr. Cassidy wrote. He added that he had spoken to Mr. Kennedy “to ensure this is not the case.”
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A few weeks later, things seemed to shift. Mr. Cassidy broke with Mr. Kennedy and called for the panel’s meeting to be delayed, saying he was concerned that the committee’s new members “lack experience studying new technologies such as mRNA vaccines, and may even have a preconceived bias against them.” But the meeting went on as scheduled.
After the firing of Dr. Monarez, Mr. Cassidy called for the panel to “indefinitely postpone” its next meeting, citing “serious allegations” about “the meeting agenda, membership and lack of scientific process being followed.” But that meeting, too, is set to go on as scheduled on Thursday and Friday in Atlanta.
Lately, Mr. Cassidy has appeared increasingly willing to take on vaccine critics including, but not limited to, Mr. Kennedy. He has sparred with the Louisiana surgeon general, Dr. Ralph Abraham, over Covid-19 vaccines, which prompted Dr. Abraham to say Mr. Cassidy should “stay in his own lane.”
He has engaged in a tit for tat on social media with a fellow Republican doctor, Senator Rand Paul of Kentucky, about whether the hepatitis B vaccine should be routinely given to infants at birth. Mr. Paul says no. Mr. Cassidy, a gastroenterologist and liver specialist who has spoken poignantly about caring for an unvaccinated young woman who died of hepatitis B, says yes.
When Mr. Kennedy canceled nearly $500 million in government grants for vaccines using mRNA technology — the same technology that produced the Covid-19 shots — Mr. Cassidy described it as “unfortunate.”
At the hearing earlier this month, Mr. Cassidy pushed Mr. Kennedy hard on the topic, arguing that his decision to narrow public health recommendations for coronavirus vaccines had made them harder to get.
Mr. Kennedy testily replied, “Is this a question, Senator Cassidy, or is this a speech you don’t want me to answer?”
Back at home, where Mr. Cassidy is facing a tough primary challenge from the right, Louisiana is enduring a public health crisis: an outbreak of whooping cough, or pertussis, that has claimed the lives of two infants.
On Friday, Mr. Cassidy posted a letter on social media saying he would work with the health secretary to address the outbreak — and publicly calling on Mr. Kennedy to promote vaccination. A spokesman for Mr. Kennedy said his department was “working closely with the State of Louisiana and its public health agency to monitor its pertussis caseload.”
Public health leaders are not holding out much hope that the health secretary will wholeheartedly endorse the whooping cough shot or any other vaccine. But as they look ahead to Wednesday, some said they hoped Mr. Cassidy’s statement was a sign that he would put public health ahead of politics.
“I hope Dr. Cassidy shows up,” said Dr. Benjamin, “and not Senator Cassidy.”
Sheryl Gay Stolberg covers health policy for The Times from Washington. A former congressional and White House correspondent, she focuses on the intersection of health policy and politics.
Megan Mineiro is a Times congressional reporter and a member of the 2025-26 Times Fellowship class, a program for early-career journalists.
