Japanese scientists have developed an oral vaccine for Alzheimer's disease that has proven effective and safe in mice, the director of a research institute behind the project said on Thursday.

The team is preparing to move to small-scale clinical trials in humans, possibly this year, said Takeshi Tabira, director of the National Institute for Longevity Sciences in Aichi, central Japan.

"We hope the Phase I trials go well," Tabira said. "Animals are able to recover their functions after developing symptoms, but humans are less able to do so. It may be that this only works in the early stages of the disease, when symptoms are light."

When administered to mice suffering from the disease, which causes dementia and is currently incurable, the vaccine reduced the amount of amyloid plaques in the brain and improved mental function.

Amyloid plaques are believed to be at the root of Alzheimer's -- a growing problem for aging populations around the world. The disease affects five million in the United States alone, the Alzheimer's Association said in a report last week.

The treatment did not cause inflammation or bleeding in the brains of the mice, Tabira said.

The vaccine is made by inserting amyloid-producing genes into a non-harmful virus. When taken orally, the virus stimulates the immune system to attack and break down the amyloid proteins in the brain, Tabira said.

The treatment was tested on 28 mice genetically modified to develop Alzheimer's disease. Half the animals were given a dose of the vaccine at the age of 10 months, while the control group were not treated.

Three months later, tests showed mental function in the treated mice had returned to levels close to those before they developed Alzheimer's symptoms.

U.S. drugmaker Wyeth and its Irish partner Elan Corp have an Alzheimer's vaccine called ACC-001 in early stage human trials.

The Japanese research, carried out in conjunction with scientists at Nagoya University and others, is to be published by the Federation of American Societies for Experimental Biology in July.