An experimental vaccine is showing promise against Alzheimer's disease, reducing brain deposits that are blamed for the disorder.

The deposits have been cut by between 15.5 percent and 38.5 percent in mice, with no major side effects, researchers said Monday in the online edition of Proceedings of the National Academy of Sciences.

Tests of the DNA-based vaccine are under way in monkeys, and if those are successful, testing in people could begin, perhaps within three years, said lead researcher Yoh Matsumoto of the Tokyo Metropolitan Institute for Neuroscience in Japan.

If all goes well, this type of treatment might be available for people in six or seven years, he said.

Scientists estimate that as many as 4.5 million Americans suffer from Alzheimer's, the most common form of dementia. The illness, named after Dr. Alois Alzheimer, a German doctor who studied it in 1906, involves the parts of the brain that control thought, memory and language.

Its cause remains unknown, and while there is no cure some drugs can slow progress of the illness in people in the early and middle stages.

Other efforts to develop an Alzheimer's therapy also are under way.

Two years ago, trials of a potential vaccine that seemed to help slow the memory decline caused by the disease were halted after several participants developed brain swelling.

Matsumoto said that problem did not occur in the tests of his vaccine.

The brains of people suffering from Alzheimer's disease are cluttered with a plaque made up of a small protein called amyloid beta, or Ab.

In the mouse tests, the Ab deposits were reduced overall, and in certain parts of the brain the reduction was as much as 50 percent, the researchers said.

Dr. Sid Gilman, a professor of neurology at the University of Michigan who has done research in vaccines for Alzheimer's, said the new research showed the vaccine was efficient and safe in the mice.

He cautioned, however, that it's very difficult to compare the level of efficiency from one trial to another, "so how effective this may be compared to other approaches is debatable."

The reasons for the brain swelling in the earlier tests have been determined, Gilman said, and new forms of that therapy are now being tested.

A variety of clinical trials are under way looking at other potential treatments for Alzheimer's including a hormone that might prevent brain cell death, several drugs, antioxidants, cholesterol-lowering medications and antibodies.

The Japanese research was funded by the ministry of education of Japan and the Novartis Institute of Biomedical Research.