Clinical trials of a vaccine for Alzheimer's Disease, which is a common and incurable cause of "senile dementia", were halted this week after it was reported that the new drug, called AN1792, had triggered a form of brain inflammation in 15 human volunteers involved in testing the drug. The vaccine was developed by the Irish pharmaceutical company "Elan" and the American company "Wyeth". When tested on mice with a similar disease, the vaccine halted, and in some-cases reversed, the characteristic Alzheimer-like changes seen in the brain. So how does the vaccine work ? It was designed to make the immune system attack protein deposits that accumulate in the brains of people with Alzheimer's Disease. These so-called beta amyloid deposits are believed to cause the disease, which affects about 1 person in 5 over the age of 80. So what went wrong ? Experts think that, in some genetically-susceptible people, the vaccine might have worked too well and stimulated an immune response that was too vigorous, producing generalised inflammation in the brain. The makers have announced that testing of this agent on humans has now been stopped.