Health and Human Services Secretary Robert F. Kennedy Jr. said Sunday that President Donald Trump wants his department to continue examining whether there is any connection between vaccines and autism, reigniting a debate that has divided public health officials for years.

Kennedy made the comments during a contentious appearance on CNN’s “State of the Union,” after host Dana Bash asked whether a Wall Street Journal report was correct in stating that Trump wanted him to investigate a possible link between vaccines and autism.

“He certainly does,” Kennedy said.

Bash questioned why HHS would pursue such research when, she said, scientists have already spent decades studying the issue and found no evidence connecting vaccines to autism. Kennedy began pointing to research involving Tylenol—which the Trump administration linked to autism late last year—as an example of the department’s broader efforts before Bash interrupted, visibly expressing frustration.

“I am because I don’t want to have you use our air to say this,” Bash said. “There have been more than 40 studies involving 5.6 million people to conclude there is no link between vaccines and autism,” she continued, citing the American Academy of Pediatrics’ Committee on Infectious Diseases.

Kennedy disputed Bash’s characterization, arguing that “none of the eight vaccines that are administered during the first six months of life have ever been studied for the relationship to autism,” while asserting that the studies Bash referenced focused on an ingredient in the measles, mumps, and rubella (MMR) vaccine.

The exchange was one of several sharp confrontations during the interview, as Kennedy and Bash repeatedly sparred over vaccine safety, public health guidance, and the government’s response to the COVID-19 pandemic. Kennedy argued that Americans’ constitutional rights were violated during the pandemic and accused Bash of contributing to misinformation through what he described as “absolute press malpractice” in covering the government’s response.

“Are you going to — you want to sit here and attack me?” Bash replied. “Or do you want to have conversations about public health?”

“Well, you’re sitting here attacking me,” Kennedy said.

Kennedy, who has long questioned vaccine policies and frequently criticized federal health officials, also used the interview to target Dr. Anthony Fauci, the former director of the National Institute of Allergy and Infectious Diseases, criticizing Fauci’s recommendations on how Americans should respond during the COVID-19 pandemic.

Since Kennedy took over as HHS secretary, federal messaging surrounding autism and vaccines has drawn increased scrutiny. The Centers for Disease Control and Prevention has revised portions of its longstanding online materials addressing claims that vaccines cause autism, changing language that had previously stated there was no causal connection.

At a Senate hearing in February, National Institutes of Health Director Jay Bhattacharya acknowledged that he has not seen research demonstrating that vaccines cause autism, a position that differs from Kennedy’s approach. Bhattacharya told lawmakers that multiple studies have found there is “no link” between the MMR vaccine and autism, while adding that other vaccines are “less well studied.”