[gdudeman]

I believe that there are studies that show that merely getting very sick increases your chance of dementia - essentially it ages you faster or brings chronic disease forward. If that's the case, vaccines for things like the flu - a disease you're likely to get - are probably good overall.

They also activate the immune system in generaly, which could probably go either way in terms of longevity.

In general I don't think vaccines are preventing so much as delaying dementia, but if they stop chronic infection they might be.