Ardent anti-vaccine activist and current US Health Secretary Robert F. Kennedy Jr. is reportedly on thin ice with Trump for not being anti-vaccine enough for the president’s liking, according to a report late Monday from The Wall Street Journal.

Trump has become fixated on the completely false, thoroughly debunked claim that vaccines cause autism, the Journal reported. People familiar with the president’s scattered thinking say Trump wants an autism remedy to be part of his legacy and thinks scaling back childhood vaccinations is a way to secure that achievement. As such, he has grown frustrated and disappointed that Kennedy hasn’t done more to cut back federal childhood vaccine recommendations or confirm a link to the neurodevelopmental disorder.

In May, the president reportedly aired his disappointment, calling out Kennedy for having “the yips” that were preventing him from acting on his zealous anti-vaccine agenda while holding the highest health position in the country. In June, Trump reportedly vented in the Oval Office to Kennedy that he wasn’t doing enough.

Kennedy has reportedly been taken aback by the criticism—and it’s likely that outside public health and medical experts would have been, too, if they were present. To experts, Kennedy has been diligently working to undermine life-saving vaccines, endangering public health in the process.

Some notable examples include Kennedy’s overhaul of a key vaccine advisory committee, which he stacked with like-minded anti-vaccine allies. Those allies then arbitrarily ditched evidence-based recommendations for the birth dose of the hepatitis B vaccine and a combination shot against measles, mumps, rubella, and varicella (chickenpox) called MMRV. Kennedy also single-handedly trimmed the US childhood vaccine schedule to resemble that of Denmark’s, a small country with universal healthcare. He directed the Centers for Disease Control and Prevention to stop promoting seasonal flu vaccines with prepaid advertisements during a particularly deadly flu season. And he edited a CDC website to falsely link vaccines to autism.