Accidental vaccines against dementia

BBC Science Focus shares an unintended consequence that’s good for us — a vaccine against shingles that has a measurable effect protecting people against dementia as they grow older:

New drugs can improve memory or slow the rate of cognitive decline in some patients, but in most cases, by the time symptoms appear, extensive and irreversible brain damage means there is no hope of a cure.

Except now, there might be. Thirteen years ago, the Welsh government unintentionally created an experiment that might just change the future of dementia care, unmasking a game-changing treatment that was there all along.

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Welsh officials introduced a new national programme on 1 September 2013, offering a shingles vaccine to people aged 70. It wasn’t a blanket rollout, it was age-targeted.

Shingles vaccines were offered in phases and the timing was largely arbitrary. If you turned 70 on or just after 1 September, you’d be offered the vaccine. If you turned 70 just before the cutoff, you were ineligible.

That seemingly insignificant detail turned out to be the most vital part of the trial – it created what’s known as a ‘natural experiment’. People on either side of the cutoff were effectively randomised, like participants in a clinical trial.

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Here’s where the payoff for dementia comes in. In 2025, Geldsetzer and other researchers sifted through the data of the Welsh septuagenarians and found something astonishing.

The people offered a vaccine were 20 per cent less likely to develop dementia.

In dementia research, 20 per cent is a big number – much higher than a lot of lifestyle factors believed to affect an individual’s risk. A follow-up study even found that the vaccine could slow the progression of the disease in people who already had it.

In other words, researchers made the near-chance discovery that this most cruel of diseases could be treated – and even prevented – by a jab in the arm. In public health terms, that’s potentially seismic.

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But the idea of a vaccine for dementia isn’t new. In the 1990s, British researchers discovered the DNA of a herpes virus in the brain cells of elderly people – specifically the common HSV-1 virus, which causes cold sores.

Later, they found that the virus was much more likely to appear in the brains of Alzheimer’s patients than people without the disease.

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Studies in multiple countries later showed that people with severe herpes infections were more likely to develop Alzheimer’s – and also that herpes patients treated with strong antivirals showed a dramatically lower risk of developing Alzheimer’s in later life.

Next came 2025 data from the US and elsewhere, suggesting a link between another herpes virus and dementia: herpes zoster, which causes chickenpox and shingles.

Researchers noticed a correlation between people who’d had the shingles vaccine and a lower risk of dementia.

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Intriguingly, however, the shingles jab is not the only existing vaccine shown to lower a person’s risk of dementia.

In 2019, researchers found that the BCG vaccine, which protects against tuberculosis, also lowers the risk of a person developing Alzheimer’s.

The team analysed data on bladder cancer patients up to eight years after treatment (the BCG vaccine is a standard treatment in bladder cancer therapy). At the same time of receiving the vaccine, none of the patients had Alzheimer’s.

In the following eight years, just 2.4 per cent of people who’d had the vaccine developed Alzheimer’s, compared with 8.9 per cent of patients who didn’t receive BCG. Those findings have since been observed in other datasets.

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Some studies have found that people who have annual flu shots see a greater benefit than those who don’t.

Likewise, in April 2026, a team at the University of Texas in the US found that a high-dose flu vaccine is linked to 20 per cent lower risk of getting dementia compared with a standard dose.

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“The first conclusion is it doesn’t really matter which vaccine you use. All are protective,” says Prof Richard Lathe, a researcher at the University of Edinburgh. “The second thing is that the more vaccines you get, the more protection you receive.”

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Lathe says the most likely mechanism is trained immunity: that vaccines don’t just prime the immune system against a specific pathogen (like herpes in the case of the shingles jab).

They also reprogramme immune cells to make the immune system better able to respond to other future challenges.