The researchers used the policy change as a natural laboratory of sorts to study the effect of shingles vaccination on long-term health outcomes. In a statistically sophisticated analysis of health records, the team found that the vaccine reduced the probability of getting dementia by one-fifth over a seven-year period. This means that people who received the shingles vaccine were less likely to develop clinical dementia over the seven-year follow-up period, and women benefited more than men.
The study design allowed researchers to compare two groups without actively depriving any one group of access to vaccination. The two groups were also of comparable age and had similar medical comorbidities – meaning similar rates of other medical conditions such as diabetes or high blood pressure.
Results from this and other related studies raise the possibility that vaccines may have a broader role in experimental therapeutics outside the realm of infectious diseases. These studies also raise provocative questions about how vaccines work and how our immune system can potentially prevent dementia.
How vaccines might be protective
One scientific explanation for the reduction of dementia by the herpes zoster vaccine could be the direct protection against the shingles virus, which may play a role in exacerbating dementia.
However, there is also the possibility that the vaccine may have conferred protection by activating the immune system and providing “trained immunity,” in which the immune system is strengthened by repeated exposure to vaccines or viruses.
The study did not differentiate between different types of dementia, such as dementia due to Alzheimer’s disease or dementia due to stroke. Additionally, researchers cannot draw any definitive conclusions about possible mechanisms for how the vaccines could be protective from an analysis of health records alone.
The next step would be a prospective, randomized, double-blind, placebo-controlled study – the “gold standard” for clinical trials in medicine – to directly examine how the herpes zoster vaccine compares with a placebo in their ability to reduce the risk of dementia over time. Such studies are necessary before any vaccines, as well as other potential therapies, can be recommended for routine clinical use in the prevention of dementia.
The challenges of untangling dementia
Dementia is a major noncommunicable disease that is a leading cause of death around the world. A January 2025 study provided updated figures on lifetime dementia risk across different subsets of the US population.
The researchers estimate that the lifetime risk of dementia after age 55 is 42% – more than double earlier estimates. The dementia risk was 4% by age 75, and 20% by age 85, with the majority of risk occurring after 85.
The researchers projected that the number of new cases of dementia in the US would double over the next four decades from approximately 514,000 cases in 2020 to 1 million in 2060.
Once considered a disease largely confined to the developed world, the deleterious effects of dementia are now apparent throughout the globe, as life expectancy increases in many formerly developing countries. While there are different forms of dementia with varying clinical manifestations and underlying neurobiology, Alzheimer’s disease is the most common.