The apparent effectiveness of an experimental Alzheimer's disease vaccine comes at a cost, according to a study reported in the March issue of Nature Medicine.

A clinical trail involving the vaccine, which is being developed by the pharmaceutical company Elan, was called off in January 2002 after six percent of the participants suffered severe brain swelling.

Afterwards, researcher James Nicoll of the University of Southampton in Great Britain, and his colleagues examined the brain of one of the female participants who took the vaccine in 2000 and later died due to a fall.

After finding that the woman had fewer plaques in her brain than Alzheimer's victims usually have, the researchers concluded that the vaccine might have cleared them out. Such plaques are believed to cause some of the symptoms of Alzheimer's disease such as forgetfulness and confusion.

However, some immune system cells normally not found in the brain of Alzheimer's patients were also found in the woman's brain. The researchers speculated that such cells were present due to an overreaction to the vaccine and could have damaged normal brain tissue.

Despite the setback, Elan hopes to begin clinical trials of a modified version of the vaccine later this year. Other experts claim that it is impossible to prove that the vaccine had any beneficial effect since the autopsied patient was not examined before taking the vaccine.

Other sources: Nature Medicine