A vaccine being used by the NHS to prevent shingles could also significantly delay the onset of dementia, according to new research that has left scientists baffled.
The study on more than 200,000 older people shows those who were given the 'recombinant' vaccine Shingrix were diagnosed with dementia an average of 164 days later than those given an older-style jab.
The effect was as pronounced as the first new drugs for Alzheimer's disease, which are currently awaiting approval by UK medicines regulators.
The scientists behind the study say they don't know what the biological mechanism is for the effect, but it is highly statistically significant.
"It is correlation, not causation," said one.
Previous studies have hinted the shingles vaccine may also have an effect on dementia, but this is by far the most significant research to date.
The researchers seized on an opportunity for what they called a "natural experiment".
In 2017, the United States switched almost overnight from using a live vaccine called Zostavax to Shingrix, which is made using genetic techniques.
Both target a virus called Herpes zoster, preventing it from flaring up in people who have previously had chickenpox.
If the virus resurfaces it causes shingles, a painful and serious condition affecting mainly older people and those with weakened immune systems.
The rapid change in vaccine allowed the researchers to compare outcomes in 200,000 people over the age of 65 before and after the date of the switch.
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By the end of the study, about the same proportion in each group had developed dementia.
But those who received their first dose of vaccine after the switchover were diagnosed with dementia on average 17% later than those who had their jab earlier.
That amounted to five and a half months.
There was a significant gender difference, with women diagnosed 22% later and men 13% later, according to results published in the journal Nature Medicine.
'Not a trivial finding'
Paul Harrison, a professor of psychiatry at the University of Oxford and one of the study authors, said: "Given the prevalence of dementia in this age group, a delay of 164 days at a public health level would not be a trivial finding.
"For individuals, if there is causality, it would feel meaningful to us."
Proving causality will be essential before the shingles vaccine could be given to prevent dementia.
What are the explanations?
"There are two obvious possible explanations for the data," said Professor Harrison.
"The first is that the herpes virus could be a risk factor for dementia, so a vaccine that stops you having re-activation of this virus might therefore be delaying whatever processes that may lead you to have dementia in the coming years.
"But the other is that this vaccine has chemicals in it to make sure your body reacts to it in a strong enough way to get the protection it seeks.
"Shingrix has a different and perhaps more potent chemical adjuvant in it than the previous vaccine.
"But we don't have any good data on this."
In the UK, the Shingrix vaccine is given to people turning 65 and those currently between 70 and 79.
Dr Maxime Taquet, who led the research, said: "The size and nature of this study makes these findings convincing, and should motivate further research."
More than 900,000 people in the UK are currently living with dementia.
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Tap hereWhat happens now?
Professor Andrew Doig, a professor of biochemistry at the University of Manchester, said: "Administering the recombinant shingles vaccine could well be a simple and cheap way to lower the risk of Alzheimer's disease.
"Now, we need to run a clinical trial of the recombinant vaccine, comparing patients who receive the vaccine with those who get a placebo. This is the most reliable way to find out how well the vaccine works.
"We also need to see how many years the effect might last and whether we should vaccinate people at a younger age.
"We know that the path to Alzheimer's disease can start decades before any symptoms are apparent, so the vaccine might be even more effective if given to people in their 40s or 50s."
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