alt Hacker News> The recent recognition of iatrogenic amyloid β cerebral amyloid angiopathy (CAA) and Alzheimer's disease, caused by inadvertent seeding of amyloid β pathology following historical medical procedures
That doesn't explain why shingles vaccinations are (somewhat) protective against Alzheimer's.
Unfortunately the vaccine-dementia association is a spurious finding. See this presentation: https://youtu.be/qlTnnQytOJ0
The mechanism is that people with the shingles vaccine are less likely to visit the hospital (because they don't get shingles). Because they have fewer hospital visits they are less likely to receive an incidental diagnosis of dementia from a hospital.