Do you have a link to these studies? I ask because, as another commenter mentioned, I think it would be very difficult to tease out cause and effect and I think the "healthy vaccinee" hypothesis could easily explain why so often you see the association between any vaccine (just here we have shingles, influenza and pneumococcal) and Alzheimer's risk reduction.

To be clear, I would get those vaccines regardless for their stated effectiveness, it just feels like so many observational conclusions ("moderate red wine is good for you!") have been proven to be non-causitive correlations when examined more closely.

Edit: Actually, the youtube link posted above talks about tons of vaccines where observational studies show reduced Alzheimer's risk: https://youtu.be/qlTnnQytOJ0?t=340