A study stopped early on safety grounds may still hold the key to a vaccine for Alzheimer's disease, say researchers.
The scientists used the body's natural defence against disease
An international team tried to boost the body's immune system by injecting patients with the beta amyloid protein that causes the disease.
The study was halted in 2002 after a some of the hundreds of patients who took part developed brain inflammation.
But two long-term follow up studies, published in Neurology, suggest many patients actually benefited.
The results renew hope that it may be possible to develop a vaccine which slows, or even prevents development of Alzheimer's.
Now a new vaccine study - which doctors hope will avoid the problems of the 2002 trial - is being launched at 30 centres in the US.
Results from the long-term follow up studies show 59 out of 300 patients who received at least one dose of the vaccine produced a significant immune response.
This group performed significantly better on a series of memory tests than those who received a dummy injection.
Brain scans also showed that their brains shrank in size - possibly because of a removal of built-up beta amyloid deposits.
And some of those who responded to the vaccine also showed decreased levels of a protein linked to brain cell death in their spinal fluid.
Dr Sid Gilman, of the University of Michigan, was one of those who stopped the original trial. He is also lead author of one of the new papers.
He said: "The idea of inducing the immune system to view beta amyloid as a foreign protein, and to attack it, holds great promise.
"We now need to see whether we can create an immune response safely and in a way that slows the progression of Alzheimer's disease and preserves cognition."
Dr Nancy Barbas, who is working on the new trial, said: "Safety is paramount, given the experience with the last trial, and the new study is designed to be extraordinarily cautious and conservative.
"But if we can show an effect, it will mean we're that much closer to giving patients and their families better options for treatment."
Rather than injecting participants with beta amyloid itself, the new trial is based on injections of antibodies produced in response to the protein.
The antibodies should help trigger the immune system to attack beta amyloid, but will be cleared by the body soon after injection.
Rebecca Wood, of the Alzheimer's Research Trust, was hopeful that immunotherapy would eventually lead to new ways to combat the disease.
But she said: "As well as finding a safe treatment, researchers need to find ways to increase the numbers of patients who actually respond to the treatment - only a fifth of people did so in the previous trial.
"More research is also needed into the actual benefits seen by patients, since there was no obvious improvement in other tests for cognitive and disability measurements."