Is there something about the shingles vaccine that reduces dementia risk?

Research suggests that somewhere in Wales right now there may be two people, born one week apart and living perhaps on the same street, whose chances of developing dementia have diverged because one got a shingles vaccine and the other didn’t. 

Could a vaccine already sitting in your GP’s fridge, recommended for an entirely different reason actually be protecting people’s brains?

Several large studies now show that people who received the shingles vaccine were less likely to develop dementia. The finding has emerged in Wales, Australia, and Canada. The evidence base has strengthened considerably in the past two years, and the signal is becoming harder to dismiss.

What study grabbed your attention most?

The Welsh study, published in Nature in 2025. What caught my attention wasn’t the finding itself, but the way it was designed.Wales launched its shingles programme in 2013 with a hard birth-date cut-off. People born on or after a specific date were eligible for a free vaccine, people born just before that date weren’t.That meant two groups, born days or weeks apart and almost identical in health and background, ended up in different vaccine-access groups purely because of an administrative calendar date. This created conditions similar to a randomised trial.

That design largely removes what epidemiologists call healthy vaccinee bias in studies comparing the vaccinated with the unvaccinated; it’s commonly argued that the vaccinated are typically more health-conscious, wealthier, better educated, or more engaged with healthcare.

When I saw about a 20 percent relative reduction in dementia diagnosis over seven years, I was intrigued.

This sounds too good to be true. What’s your assessment of the evidence?

I am intrigued. The same quasi-experimental approach was applied and replicated in Australia, published in JAMA, and then in Ontario, Canada, published in The Lancet Neurology. Three countries and three health systems, and three sets of findings all using natural experiments created by different vaccine eligibility rules all pointed in the same direction.

Would it still be premature to say the shingles vaccine, designed to prevent the reactivation of the varicella-zoster virus, reduces the risk of dementia?

This has moved from an interesting observational association to a plausible causal hypothesis. We’re not at the point where we can say the shingles vaccine prevents dementia.

Three natural experiments in different health systems have produced broadly concordant results using the older live shingles vaccines. That makes a causal effect plausible.

We don’t know the precise size of the effect, whether it operates mainly by preventing shingles or through broader immune mechanisms. We also don’t know if the newer more effective vaccine Shingrix (used in New Zealand) produces the same effect.

But GSK, one of the world’s largest vaccine manufacturers, has launched a trial of 33,600 participants in Finland, specifically designed to assess whether Shingrix reduces dementia risk in adults aged 76 and over. It will not report for several years, but it represents a serious commitment to finding out.

The Welsh study found a particularly strong effect in women. Was that unique to Wales?

This is one of those findings that keeps popping up just often enough to be hard to ignore. The apparent benefit was much stronger in women in Wales. Ontario pointed in the same direction, although the evidence wasn’t quite strong enough to be definitive. Australia, meanwhile, didn’t see a difference between men and women at all. So, the signal is there, but it’s still fuzzy.

There is a repeated suggestion of a larger effect in women, with some striking individual estimates, but inconsistent formal evidence of effect modification across studies. It’s worth watching and worth researching properly but not yet established.

What is the most plausible explanation for how the vaccine could influence dementia risk?

We do not yet know, and there may be more than one mechanism at work.

One possibility is that preventing reactivation of the shingles virus reduces episodes of inflammation, vascular injury, or other neurological effects that could contribute to cognitive decline over time. Another is that vaccination alters immune responses in ways that are beneficial to the ageing brain more broadly.

An intriguing 2025 study found that an RSV vaccine containing the same adjuvant system (immune enhancer) as Shingrix showed a similar signal, despite targeting a completely different virus. The interpretation has been contested and mechanism remains unresolved, but it adds to the sense that something interesting is happening here beyond simple viral prevention.

What does this mean for New Zealanders, given we use Shingrix and funding is limited to a narrow age window?

The strongest evidence so far comes from studies of Shingrix. A 2024 study in Nature Medicine found people who received Shingrix spent about 17 percent longer without a dementia diagnosis than those given the older vaccine, Zostavax, equivalent to about 164 extra dementia-free days among those later affected.

Another large study, published this year and involving nearly 66,000 Shingrix recipients, found much the same thing. These studies point the same way: the emerging evidence is directly relevant to the vaccine New Zealand now uses.

But the funding window is unusually narrow in New Zealand. For most people, Shingrix is funded only at age 65 – not at 64, and not if you’re 66. It’s a window that is easy to miss. If you don’t know you’re eligible, aren’t offered it, or simply don’t engage with healthcare during that year, that’s an opportunity lost. After that, the two-dose course costs $600 to $800.

That raises an equity question regardless of what the dementia research ultimately shows. Outside the window, access depends on people’s ability to pay.

What would you say to someone who thinks: should I get vaccinated to protect my brain?

If you are over 50 and haven’t had both doses of the shingles vaccine, you already have an excellent reason: it is highly effective at preventing shingles, which causes significant pain and serious complications. That’s a very good reason to get one.

The dementia evidence is additional context. It is credible and increasingly consistent, and the evidence keeps accumulating. I wouldn’t say the shingles vaccine prevents dementia because the evidence does not support that claim yet. But the evidence that it may also be doing something beneficial for brain health is getting stronger. You’re entitled to know that, and it’s worth raising with your doctor.