A revolutionary drug that stops Alzheimer's disease in its tracks could be available within a few years.

It could prevent people from reaching the devastating final stages of the illness, in which sufferers lose the ability to walk, talk and even swallow, and end up totally dependent on others.

The jab, which is now being tested on patients, could be in widespread use in as little as six years.

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The most common cause of dementia, Alzheimer's affects around 500,000 Britons, with about 500 new cases diagnosed every day as people live longer.

Treatment costs the NHS up to £14billion a year - more than it spends on strokes, heart disease and cancer combined.

Existing drugs can delay the progress of the symptoms, but their effect wears off relatively quickly, allowing the disease to take its devastating course. In contrast, the new vaccine may be able to hold the disease at bay indefinitely.

Professor Clive Ballard, of the Alzheimer's Society, said: "A successful vaccine would be a groundbreaking treatment advance for the 25million people with Alzheimer's disease worldwide."

Vaccines are typically used to provide immunity to a disease as a preventive measure before it can develop, but this is an example of a therapeutic vaccine, used to treat a disease which has already developed.

Known as CAD106, it is the brainchild of scientists at Zurich-based biotechnology firm Cytos, which is also developing anti-smoking, obesity and flu vaccines.

Cytos chief executive Dr Wolfgang Renner said: "If it could prevent the progression of Alzheimer's, it would be fantastic."

Early tests showed the vaccine is highly effective at breaking up the sticky protein that clogs the brain in Alzheimer's, destroying vital connections between brain cells.

When the jab was given to mice suffering from a disease similar to Alzheimer's, 80 per cent of the patches of amyloid protein were broken up.

The vaccine is now being tried out on 60 elderly Swedish patients in the early and middle stages of Alzheimer's. Half of the men and women are being given the vaccine while half are being given dummy jabs.

Although the year-long trial is designed to show that the treatment is safe, the researchers will also look at its effect on the patients' symptoms.

While the results are not due until early next year, the initial findings are promising. Dr Renner told a Zurich conference earlier this week: "I am glad to report that the vaccine is very well tolerated."

If the trial is successful, larger-scale trials will follow, in which researchers will work out the best dose to give and how often it should be given. The finished product is six to eight years from the market.

The vaccine uses a tiny section of the amyloid protein attached to an empty virus shell to trick the immune system into attacking and breaking up deposits of protein clogging the brain.

Scientists at Cytos, who have sold the rights to the vaccine to Swiss pharmaceutical giant Novartis, say the vaccine is likely to be given to those in the early stages of Alzheimer's, to stop the disease from progressing.

The development of tests capable of detecting the disease in its earliest stages would allow the jab to be given at the first possible opportunity.

It could also be used to keep the disease at bay in those with a strong family history of the illness, and even for the mass vaccination of people in late middle age.

However, while the jab may stop the disease in its tracks, it is not expected to repair dead tissue, and so will not be a cure. Nevertheless, preventing the disease's progression would have an enormous impact on sufferers' lives.

British Alzheimer's experts welcomed the research.

Professor Ballard of the Alzheimer's Society said a previous development, the Elan jab, which failed due to severe side-effects, "was seen as the most exciting treatment development ever investigated for the treatment of dementia".

He added: "This new vaccine is potentially promising as scientists hope they have found a way to limit adverse reactions."

Rebecca Wood, of the Alzheimer's Research Trust, said the vaccine is still many years away from the pharmacy shelf.

But she added: "This is very exciting research and brings us a step closer to finding an answer to this terrible disease.

"With 700,000 people with dementia in the UK - a number forecast to double within a generation - we urgently need to find ways to halt this devastating disease."

Experts forecast that the ageing population will result in a "global epidemic" of Alzheimer's, with one in 85 people being sufferers by 2050.