Evidence exists for herpesvirus infection to increase the risk of suffering Alzheimer's disease. The evidence is mixed, however, with some studies showing no effect. There is some suggestion that the interaction of multiple viruses is the real contributing effect, which is why looking at just one virus type may produce problematic data. Researchers here show that vaccination against herpes zoster reduces Alzheimer's risk, though one might as to whether this is because of reduced viral impact, or because vaccinations can produce a trained immunity effect, reducing chronic inflammation in older people.
There is growing interest in the question if infectious agents play a role in the development of dementia, with herpesviruses attracting particular attention. To provide causal as opposed to merely correlational evidence on this question, we take advantage of the fact that in Wales eligibility for the herpes zoster vaccine for shingles prevention was determined based on an individual's exact date of birth. Those born before September 2 1933 were ineligible and remained ineligible for life, while those born on or after September 2 1933 were eligible to receive the vaccine. The percentage of adults who received the vaccine increased from 0.01% among patients who were merely one week too old to be eligible, to 47.2% among those who were just one week younger. No other interventions used the exact same date-of-birth eligibility cutoff as was used for the herpes zoster vaccine program.
This unique natural experiment allows for robust causal, rather than correlational, effect estimation. We show that receiving the herpes zoster vaccine reduced the probability of a new dementia diagnosis over a follow-up period of seven years by 3.5 percentage points, corresponding to a 19.9% relative reduction in the occurrence of dementia. Besides preventing shingles and dementia, the herpes zoster vaccine had no effects on any other common causes of morbidity and mortality. In exploratory analyses, we find that the protective effects from the vaccine for dementia are far stronger among women than men. Our findings strongly suggest an important role of the varicella zoster virus in the etiology of dementia.