Shingles Vaccines reduce risk dimentia... because

 https://www.sciencealert.com/new-shingles-vaccine-does-even-better-job-of-delaying-onset-of-dementia

From the article, bold emphasis by me:

But it may not be that the vaccines themselves protect against dementia. Rather, it may be the resulting lack of viral infection creating this effect. Research indicates bacterial infections in the gut, as well as viral infections, are associated with a higher risk of dementia.

Notably, untreated infections with herpes simplex (herpes) virus – closely related to the varicella-zoster virus that causes shingles – can significantly increase the risk of developing dementia. Research has also shown shingles increases the risk of a later dementia diagnosis.

The mechanism is not entirely clear. But there are two potential pathways which may help us understand why infections could increase the risk of dementia.

First, certain molecules are produced when a baby is developing in the womb to help with the body's development. These molecules have the potential to cause inflammation and accelerate ageing, so the production of these molecules is silenced around birth. However, viral infections such as shingles can reactivate the production of these molecules in adult life which could hypothetically lead to dementia.

Second, in Alzheimer's disease, a specific protein called Amyloid-β go rogue and kill brain cells. Certain proteins produced by viruses such as COVID and bad gut bacteria have the potential to support Amyloid-β in its toxic form. In laboratory conditions, these proteins have been shown to accelerate the onset of dementia.