I am not aware of what was the case historically (roughly before I was vaccinated as a kid), but I will point out the switch happened before covid- around 2015- and was fairly partisan. Note that extreme idealogies sort of "wrap around" the political spectrum- another area with extremely low vax rates for some time was Marin, which is highly educated and liberal.

I wonder to what extent any "partisan switch" is primarily a US thing? Here in Australia, while some right-leaning people (especially the "hard" right) have in recent years become "anti-vaccine", I haven't seen any comparable evidence of traditional left-leaning "anti-vaccine" adherents abandoning it

I can think of two people my wife and I know personally who are vocally "anti-vax" – one is a rather conservative Christian, the other is a woman with a long-standing interest in the New Age / Tarot / occult / crystals / psychics / alternative medicine / etc. The first, when I was younger (15–20 years ago), I used to run in his circles a lot, and I never heard anybody say anything remotely against vaccines at any of the churches / youth groups / bible studies / etc I used to go to with him. He only started becoming "anti-vax" around when COVID started, and it obviously had a lot to do with US politics (he was a big Trump fan, and so was his church–I'm not sure if they still feel the same way about him now), despite living on the other side of the planet. Even then, he was always primarily "anti-COVID vax" rather than "anti-vax" in general.

Our "New Age" friend, her being "anti-vax" was entirely expected, because "anti-vax" opinions have always been very common among people like her. She became very political about it around COVID, organising protests, etc. It became even more intense when she lost her government job (as an adult vocational education teacher) due to her refusal to be vaccinated against COVID. But, the anti-vax protest movement she became involved in was rather left-leaning, it was all composed of New Age types, hippies, etc, who live in a "hippy area" (the Northern Rivers district of New South Wales). I think if someone like our other friend – a conservative Christian spouting pro-Trump rhetoric – had turned up at one of her protests, she would have run them off.

> Note that extreme idealogies sort of "wrap around" the political spectrum

I don't know if that's really true though. While opposition to COVID vaccination may be one of the very few things these two friends of ours could agree on, I think their reasons for their respective positions are very different. I'm not sure the whole "wrap around" or "horseshoe" thesis is really that useful in understanding why two people from opposite ends of the political spectrum may occasionally end up agreeing on something, even something outside the mainstream.

Yeah, I remember an episode of The Simpsons circa 2016 (so they probably wrote it in 2015) where they call it out as a "liberal" thing.