Republican senator Bill Cassidy – a medical doctor who voted to confirm anti-vaccine activist Robert F Kennedy as federal health secretary in early 2025 – on Sunday denounced Donald Trump’s “crazy, stupid” claims about the safety of the combined MMR vaccine for undermining faith in immunization.

When Trump recently ordered changes to the recommended childhood vaccination schedule, with Kennedy by his side, the president called for two shots of the combined Measles, Mumps and Rubella vaccine to be scrapped in favor of two doses of three separate vaccines, claiming that the MMR shots are “sort of like a nuclear weapon” that could be “quite lethal”.

Cassidy, whose re-election bid ended in defeat to a Trump-backed challenger in a Republican primary in Louisiana, has been on the president’s enemies list since 2021, when the senator voted to convict him of “incitement of insurrection” in connection with the US Capitol attack carried out by Trump supporters toward the end of his first presidency.

Asked about Trump’s comment that “nothing bad can happen” from advising parents to get six separate shots for their children, instead of two, Cassidy told CNN on Sunday: “If it wasn’t so potentially tragic, you would break out laughing at a comment like that.

“Let’s just take it from the perspective of the parent: now instead of going to the doctor twice to get their child vaccinated, they have to go six times. Instead of the child getting two jabs, they get six jabs,” he continued. “Instead of the insurance company paying for two doctor’s visits, they’re paying for six, which is going to ultimately drive up premiums …

“So the president is wrong; there’s a lot of bad things that will come out of this.”

Cassidy’s critics have lambasted the outgoing senator for expressing misgivings about Trump’s actions, including a recent shouting match over the ongoing Iran war – but continuing to cast crucial votes to confirm the president’s nominees.

He cast the decisive vote on 8 August that allowed Todd Blanche – a former criminal defense lawyer for Trump – to secure Senate confirmation as US attorney general.

When Cassidy, the chair of the Senate health committee, previously announced that he would vote to confirm Kennedy as health secretary, he acknowledged that “the most notable opponents of Mr Kennedy were pediatricians on the frontlines of our children’s health who regularly have to combat misinformation; combating vaccine skepticism with correct information – correct information that comes from their education, training and experience as physicians.”

Cassidy also revealed that he had extracted a promise from Kennedy, who has pushed the discredited theory that vaccines cause autism for decades, that “CDC will not remove statements on their website pointing out that vaccines do not cause autism”.

After Kennedy became health secretary, however, the “Autism and Vaccines” page on the CDC website was altered to undermine that scientific consensus. An asterisk was added to the headline “Vaccines do not cause Autism”, which leads to a footnote pointing out that “the root causes of autism” remain unknown.

Text was also added to the top of the same page, so that anyone who searches for CDC guidance on autism and vaccines is now presented with two “key points” that further appease vaccine skeptics and conspiracy theorists.

“The claim ‘vaccines do not cause autism’ is not an evidence-based claim because studies have not ruled out the possibility that infant vaccines cause autism,” the first key point reads.

The second addition, since one of the nation’s leading anti-vaccine activists took charge, suggests that the lack of evidence for a link between vaccines and autism could be the result of a conspiracy. “Studies supporting a link have been ignored by health authorities,” the CDC site now claims.

Cassidy on Sunday told CNN that Trump’s MMR commentary “will do nothing” to address the discredited link between vaccine and autism stemming from a single paper that was ultimately retracted.