The US Centers for Disease Control and Prevention has recast the vaccine safety section of its website to align with the view of Health Secretary Robert F. Kennedy Jr. that childhood vaccines cause autism, countering decades of science showing them to be safe.

The US public health agency's website was changed to say, "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."

It added that health authorities have "ignored" studies supporting the link between the two.

Public health experts, doctors and scientists decried the update as the kind of misinformation the CDC has fought for decades as it promoted the use of life-saving childhood vaccines both in the US and abroad.

Until Kennedy, a longtime anti-vaccine proponent, took up his role as head of Health and Human Services, the CDC was a key opponent of growing global anti-vaccine sentiment.

Some of that can be traced to a now discredited 1998 study that linked the measles vaccine and autism. President Donald Trump has also expressed anti-vaccine sentiments.

The CDC's website previously said "studies have shown there is no link between receiving vaccines and developing autism spectrum disorder."

The World Health Organization and other health agencies around the world have repeatedly said evidence shows vaccines do not cause autism and referred back to earlier statements when asked about the CDC website change on Thursday.

"This represents a new and devastating turn by the CDC, which has been effectively dismantled by the Secretary of HHS," said Helen Tager-Flusberg, an autism expert at Boston University.

The website change happened without the consultation of CDC staff who were studying autism, said one CDC scientist who requested anonymity because they were not authorised to speak to reporters.

Andrew Nixon, a spokesperson for HHS, defended the change and did not address questions on who had ordered it.

Kennedy cleared the way for CDC policy changes in August, when he fired Director Susan Monarez over vaccine policy. The agency is now led by acting director and deputy HHS Secretary Jim O'Neill, who is not a scientist.

Scientists took issue with statements on the website that studies have not ruled out the possibility that infant vaccines cause autism, arguing that it is "exploiting a quirk of logic."

"You can't prove something never happens," Jake Scott, a professor at Stanford Medical School, wrote on Substack. "Scientists can't prove vaccines never cause autism because proving a universal negative is logically impossible."

Jesse Goodman, a former FDA chief scientist, said the website now ignores multiple large, well-done studies that have shown no association of vaccines with autism.

The studies it cites "have major flaws and do not control adequately for other factors potentially associated with autism diagnoses," he said. In particular, he and others cite a large, landmark 2019 Danish study.

The CDC website cites a 2012 review done by the Institute of Medicine as saying that all but four studies of the relationship between the Measles-Mumps-Rubella vaccines and autism had "serious methodological limitations."

It did not include that review's conclusion that the evidence nonetheless favours rejection of a causal relationship between the MMR vaccine and autism.

Autism is a neurological and developmental condition marked by disruptions in brain signalling that cause people to behave, communicate, interact and learn in atypical ways. The causes of autism are unclear.