The Centers for Disease Control and Prevention puts the U.S. death count from Covid-19 at 1.2 million. But Health Secretary Robert F. Kennedy Jr. rejects his own department’s statistics.

“I don’t know how many died,” Mr. Kennedy told senators during a contentious hearing Thursday, adding, “I don’t think anybody knows, because there was so much data chaos coming out of the C.D.C.”

When Senator Elizabeth Warren, Democrat of Massachusetts, said, accurately, that it was becoming harder for people to get Covid shots at pharmacies, Mr. Kennedy denied that, retorting: “Everyone can get access to them.” He also claimed mRNA vaccines “cause serious harm, including death, particularly in young people,” though the C.D.C. found “no increased risk of death” from the shots.

Mr. Kennedy’s defiant performance at the Senate Finance Committee on Thursday, his critics say, put on vivid display what happens when someone with no medical or scientific training and a conspiracy-minded approach takes charge of the nation’s public health.

Instead of being guided by rigorous research and nuanced debate over complex issues that defy easy answers, the country’s health secretary rejects facts that do not fit his theories and casts out experts who are not aligned with him. He is buoyed by the support of a vocal populist base that shares his suspicions of organized medicine and is energized by his push for a new approach.

Mr. Kennedy’s distrust of his department’s scientists and doctors has created turmoil within the nation’s public health agencies, particularly the C.D.C. Mass firings and restructuring have stripped the institution of expertise. And the health secretary dismissed an entire panel of vaccine experts and fired the C.D.C. director — whom he had called a “brilliant microbiologist and a tech wizard” — just one month after the Senate confirmed her.