In the days leading up to a pivotal vote on Robert F. Kennedy Jr.'s nomination to become America's top heath official, all eyes were focused on Sen. Bill Cassidy. And the pivotal issue for the Louisiana Republican was Kennedy's skepticism toward vaccines.

Cassidy chairs the Senate committee that oversees the Department of Health and Human Services. He is also a doctor who is proud of his work to get tens of thousands of children vaccinated against hepatitis B, a disease that can lead to liver failure, cirrhosis and cancer. In a nomination hearing last month, he told Kennedy he was afraid people would die of vaccine-preventable diseases "because of policies or attitudes that you bring to the department."

Yet Cassidy ultimately provided the one-vote margin needed to advance Kennedy's nomination to the full Senate. He said he was swayed by Kennedy's commitments to support the immunization schedules recommended by the Centers for Disease Control and Prevention, maintain systems used to vet new vaccines and monitor their safety, preserve statements on the CDC website assuring the public that vaccines don't cause autism and meet with Cassidy "multiple times a month," among other things.

"I will watch carefully for any effort to wrongfully sow public fear about vaccines," said the senator, who is up for reelection in 2026.

Cassidy's intent to serve as a one-man bulwark against measles, polio and dozens of other ailments may be sincere. But in reality, he will have little power to enforce the terms of his agreement with Kennedy, experts said.

Kennedy, if confirmed, will oversee more than 80,000 workers in the CDC, the Food and Drug Administration, the National Institutes of Health and the Center for Medicare and Medicaid Services, among other agencies.

Cassidy, on the other hand, controls just one of the 100 votes in the Senate.

His power to ensure that the would-be Health and Human Services leader would not merely tolerate vaccines but actively promote them was greatest before last week's committee vote, when he had the leverage to derail the nomination. Cassidy could still change his mind before the final Senate vote — a move that would be welcomed by some medical and patient advocacy groups — but at least three other Republicans would have to join him to prevent Kennedy's confirmation

He will have even less influence after Kennedy is installed at the health department, said Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania.

"While I appreciate what Cassidy is trying to accomplish, his capacity to act once the confirmation goes through is limited," Jamieson said. "Do I expect he will do everything he can? Yes. Do I think it's going to be productive with Robert Kennedy given everything we know about him with respect to vaccination? No."

Dr. Georges Benjamin, executive director of the American Public Health Association, was also pessimistic about Cassidy's prospects for making sure Kennedy upholds his side of the bargain. He recalled that Sen. Susan Collins (R-Maine) said she had received assurances from Neil Gorsuch and Brett Kavanaugh that they wouldn't overturn Roe vs. Wade if they were elevated to the Supreme Court.

"You see how that worked out," Benjamin said.

Cassidy declined to elaborate on his agreement with Kennedy, according to a spokesperson for the Senate Health, Education, Labor and Pensions Committee.

Even if Kennedy follows through with all of the items in the deal, there are myriad other actions he could take that would be at odds with its spirit, Jamieson said.

For instance, she said, the CDC could post a warning on its website advising people not to take mRNA vaccines. That scenario is not so far-fetched — Florida's surgeon general has discouraged the public from getting COVID-19 booster shots based on unscientific concerns about how mRNA vaccines work. (The same health official also advised Florida parents whose unvaccinated children attended a school with an active measles outbreak that they didn't necessarily need to keep their kids at home for the recommended 21 days.)

Anti-vaccine information on federal government websites could spread far and wide since the overview results for Google searches tend to draw on information from the CDC website, Jamieson said.

Should Kennedy or anyone else at the Department of Health and Human Services take steps that undermine support for vaccines, Cassidy could flex his authority as the HELP Committee chairman and make good on his threat to call an oversight hearing.

"There's no cabinet secretary who welcomes an appearance before an oversight committee," said Dr. Peter Lurie, president of the Center for Science in the Public Interest, a nonprofit focused on public health issues.

When Lurie worked in the FDA during the Obama administration, such hearings were dreaded events that required "hours and hours" of preparation, he said.

"They're intimidating," he said, and government officials have been known to make deals to get out of them.

With a seat on the closely divided Finance Committee, Cassidy could also threaten to oppose Kennedy's spending priorities in future votes, Lurie added.

During his second nomination hearing, Kennedy offered to have a deeper discussion with Cassidy about the research that's convinced the mainstream scientific and medical community that vaccines do not cause autism.

"You show me those scientific studies and you and I can meet about it," Kennedy said.

It's possible the men could use their regular get-togethers for this purpose, but it's unlikely to break the impasse, Jamieson said.

"Anything Cassidy says is going to be futile," she said. Kennedy "can adopt radical skepticism about anything."

A more productive use of that time would be to "find common ground" by focusing on ways to support initiatives like the National Diabetes Prevention Program and the Health Center Program that both improve healthcare and lower costs, said Dr. Elbert Huang, director of the Center for Chronic Disease Research and Policy at the University of Chicago.

Dr. James Krieger, executive director of the advocacy group Healthy Food America, suggested the two could work together to close the legal loophole that allows food companies to add new compounds to their products without notifying the FDA. They could also tackle other problems, like reducing corporate influence on government health decisions and collaborating with the Federal Trade Commission to restrict junk food ads that target children.

"People have been talking about this stuff for a long time," Krieger said.

Cassidy has been clear all along that he supports Kennedy's focus on preventive health. Whether that will be enough to make up for his vaccine views remains to be seen.

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