Found it! I think this poster really nailed it.
We're research scientists with MDs and PhDs and NO, we personally know (or have heard of in our circle) absolutely zero PhDs or MDs who are vaccine-deniers.
My aunt, who has a high school diploma, became a raw vegan, and vaccine denier, decades ago. She died of Covid last year.
She was intelligent, but with a very contrarian, anti-establishment sort of personality. She felt the government was hiding things from her, the medical establishment was under the control of Big Pharma, etc. She lacked the critical thinking and scientific approach necessary to distinguish false experts from legitimate ones. Despite our efforts at providing information, she did not deviate from her beliefs, and died for them.
I think some human brains are just wired to find satisfaction in countering the established knowledge and power centers. They're people who feel stifled by the mundanity of their daily lives, and feel compelled to seek more exciting/grandiose explanations that place them in more central roles. Some may be twice exceptional (high IQ with ADHD/ASD/learning disabilities) who are underemployed and lack constructive outlets for their splinter skills.
https://www.dcurbanmom.com/jforum/posts/list/15/1170151.page