Swedish doctors reported the results Wednesday, saying that the way is now open for wider trials.

The prototype vaccine, called CAD106, is a new exploration of traditional vaccine engineering - pathogens that cause diseases are used to teach the immune system to identify an intruder and attack it.

In Alzheimer's, one of the enemies is a toxic protein called amyloid beta peptide, which accumulates in plaques in the brain, although exactly how it works remains unclear.

Ten years ago, doctors launched the first attempt at an amyloid beta vaccine, called AN1792. But they were forced to abandon it after 6% of the volunteers fell ill with meningoencephalitis, an inflammation of the brain. The suspected reason was that AN1792 activated T cells that attacked the brain tissue.

The new vaccine (CAD106) uses a smaller fragment of the protein and combines it with a booster to prevent T-cell activation.

A team led by Bengt Winblad of the Karolinska Institutet's Alzheimer's Disease Research Centre, tested the vaccine on 46 volunteers aged 50 to 80, diagnosed with mild to moderate Alzheimer's. A "control" group of 12 patients received a placebo. After a year, 82% of those who received the drug developed antibodies, a sign that their immune defenses had responded to the dangerous protein.

The next step should be a larger test, possibly with modifications of the dose, to see if the vaccine works, reported the researchers.

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