How Routine Vaccines May Reduce Dementia Risk: The Trained Immunity Hypothesis

When Marco Fernandez rolled up his sleeve for his second Shingrix shot last fall, he wasn't thinking about his brain. The 72-year-old retired teacher was focused on avoiding another bout of shingles, that agonizing nerve pain that had sidelined him for months. But what if that same vaccine was quietly protecting his mind?

That question sits at the center of one of the most surprising developments in dementia research. A growing body of evidence suggests that routine vaccines may do far more than prevent infections. They appear to reduce the risk of cognitive decline, and scientists now have a leading hypothesis for why: something called trained immunity.

What Is Trained Immunity?

Your immune system has two branches. The adaptive branch is the one you might recognize from school, it learns specific pathogens and creates memory cells that recognize them the next time. But there's another branch, the innate immune system, that was long thought to be more rigid and fixed.

That view changed around 2011, when researchers demonstrated that the innate immune system could be trained. A landmark 2012 study published in PNAS showed that BCG vaccination, the tuberculosis vaccine given to millions of children worldwide, enhanced innate immune responses in both mice lacking adaptive immunity and in humans [2]. BCG-primed human blood cells produced stronger responses not only to tuberculosis but also to unrelated pathogens like Candida albicans and Staphylococcus aureus. The effect wasn't specific to one germ; it was broad.

The mechanism involves epigenetic reprogramming. After certain stimuli, innate immune cells like monocytes, macrophages, and natural killer cells undergo chemical changes that alter gene activity without changing the underlying DNA sequence [1]. These epigenetic tags make genes coding for pro-inflammatory signals more active, so when the body encounters a future threat, the response is faster and more intense. It's as if the immune system has been doing morning drills, better prepared for unexpected encounters.

The Dementia Connection Emerges

The connection between vaccination and dementia protection has been accumulating for years. A 2022 meta-analysis in the Journal of Alzheimer's Disease found that flu vaccination was associated with a 40% lower risk of developing Alzheimer's disease over four years in people over 65 [5]. The pattern that emerged was striking: the more frequently someone received the flu shot, the greater their protection appeared to be.

This dose-dependent response is crucial. It suggests something biological rather than simply that healthier people are more likely to get vaccinated. If vaccines were merely marking people who took better care of themselves, you wouldn't expect more shots to correlate with more protection. But that's exactly what the data shows.

The strongest evidence has accumulated around the shingles vaccine. A large Kaiser Permanente study published in Nature Communications found that two doses of the recombinant zoster vaccine (RZV/Shingrix) was associated with a 51% reduction in dementia risk in adults aged 65 and older [3]. The cumulative dementia incidence was 5.67% in vaccinated individuals compared to 10.64% in an unvaccinated comparison group over a mean follow-up of about 3.4 years. The protection was similar across age groups, racial and ethnic groups, and types of dementia including Alzheimer's disease and vascular dementia.

Crucially, the protective effect persisted even in people who never developed herpes zoster during the follow-up period [3]. That matters because it suggests the vaccine isn't simply preventing shingles and its potential neurological complications, something else is going on.

The AS01 Adjuvant: A Shared Feature

When researchers at the University of Oxford analyzed data from more than 436,000 people, they found that both the shingles vaccine and the RSV vaccine (Arexvy) were associated with reduced dementia risk compared to the annual flu vaccine [4]. The shingles vaccine showed an 18% reduction; the RSV vaccine showed a 29% reduction; people who received both had a 37% reduction. What do these two vaccines have in common that the flu vaccine lacks? They both contain the AS01 adjuvant.

The AS01 adjuvant contains monophosphoryl lipid A, a component derived from bacterial cell walls that activates macrophages and dendritic cells and triggers interferon gamma production [1][4]. In mouse models, this interferon gamma response has been shown to reduce amyloid plaque deposition, those sticky protein aggregates that accumulate in Alzheimer's disease [4]. Interferon gamma levels are negatively correlated with cognitive decline in cognitively unimpaired older adults [4].

The Oxford researchers noted something telling: despite targeting completely different viruses, the two AS01-adjuvanted vaccines showed no significant difference in dementia protection when compared to each other [4]. That convergence supports the idea that the adjuvant, not the pathogen, is driving the effect.

The protective signal appeared within 18 months of vaccination, too quickly to be explained by infection prevention alone [4]. If preventing shingles or RSV was the mechanism, you'd expect the benefit to build over years as infections were avoided. Instead, the effect materialized relatively fast, consistent with an immunological mechanism like trained immunity.

The Broader Pattern

Eight vaccines have now been linked to reduced dementia risk: shingles, RSV, flu, DTP (diphtheria-tetanus-pertussis), pneumococcal, hepatitis A, hepatitis B, and typhoid [5]. The evidence is strongest for shingles, flu, RSV, pneumococcal, and DTP vaccines. A meta-analysis published in Age and Ageing in 2025, covering 104 million participants, found a 33% reduction in dementia risk associated with adult vaccination [5].

This breadth is itself informative. If only one or two vaccines showed protection, coincidence or confounding could explain the findings. But when multiple vaccines across different pathogens point in the same direction, something fundamental appears to be at work.

The 2026 Frontiers in Immunology hypothesis paper proposed that non-specific effects of vaccination, operating through trained innate immunity, provide a mechanistic explanation for these observations [1]. The paper notes that herpesviruses including HSV and VZV, flaviviruses, and SARS-CoV-2 have all been linked to increased dementia risk, suggesting that some dementia cases may have an infectious disease component [1]. Uncontrolled or excessive neuroinflammation appears to be a central parameter upon which dementia risk from infections, vaccines, and anti-inflammatory drugs mechanistically converges.

Implications for Your Health Decisions

This research doesn't yet change clinical recommendations. No major health authority has endorsed vaccination specifically to prevent dementia. The studies show association, not causation, and the healthy vaccinee effect, the possibility that people who get vaccinated are systematically different from those who don't, remains an important caveat.

But the mechanistic evidence for trained immunity is now solid, and the epidemiological pattern is consistent across multiple vaccines and populations. The authors of the Frontiers in Immunology paper state explicitly that elucidating these mechanisms may open new avenues to promote healthy aging through vaccination [1][2].

For now, the practical implication is straightforward: staying up to date with routine adult vaccinations may offer cognitive benefits beyond their intended targets. The shingles vaccine has accumulated the most replicated evidence, and it's recommended for adults 50 and older regardless of whether they've had chickenpox. The flu vaccine is recommended annually for most adults. The RSV vaccine is now available for older adults in many countries.

These are not primarily dementia-prevention vaccines. But as you weigh whether to schedule that next shot, the evidence suggests your brain may be among the beneficiaries.