> they're not sure if the vaccine will last in your system for more than 30 years

Vaccines don't really last in your system more than a few days (although some cells get dragged off to the lymph nodes and get harshly interrogated for longer than that). It is the T-cell and B-cell responses that are persistent for years.

> and that subsequent doses may be less effective

They should activate B cells that you already have and produce high-affinity mature antibodies.

It's a fair callout, but for me (and increased propensity for Alzheimer's) the calculus is a bit different. Making to to my mid-70s with full mental faculties would easily be worth the tradeoff of an increasing likelihood of shingles at that point. And besides, I would presume I would be able take the vaccine again?