CHICAGO/LONDON (Reuters) - An experimental vaccine helped monkeys with a form of the AIDS virus control the infection for more than a year, suggesting it may lead to a vaccine for people, U.S. researchers said on Wednesday.

They said the vaccine works by priming the immune system to quickly attack the HIV virus when it first enters the body, a point at which the virus is most vulnerable.

Dr. Louis Picker of the Oregon National Primate Research Center, whose study appear in the journal Nature, said he thinks it will be possible to have a vaccine ready to test in people within three years.

Tests of the vaccine with a primate version of the virus called simian immunodeficiency virus showed more than half were able to keep the virus from replicating so that even the most sensitive tests could not detect any traces of the virus.

So far, the vast majority of the vaccinated monkeys have maintained control over the virus for more than a year, gradually losing any signs that they had ever been infected.

Macaques in the unvaccinated group have since developed the monkey form of AIDS.

"We feel it has a possibility of keeping the virus under complete control or clearing the virus," Picker said.

Picker and colleagues use a relatively harmless virus called cytomegalovirus (CMV) as a transport system to take the experimental vaccine into the body.

They chose it because scientists think most people are already infected with CMV -- a virus that remains in the body for life but causes little or no symptoms for most people.

Picker said because the virus is persistently present, it keeps the immune system on alert, ready to attack the virus as it first enters the body, when the virus is thought to be less impervious to the immune system.

"The virus comes in and can be basically stopped in its tracks," Picker said in a telephone interview.


"What's exciting about these findings is that for the first time a vaccine candidate has been able to fully control the virus in some animals," said Dr Wayne Koff, chief scientific officer at the International AIDS Vaccine Initiative (IAVI), which helped fund the research.

Koff said the findings also suggested the possibility that the immune system may eventually eliminate the virus altogether.

"This research gives us potential clues as to how we might design an HIV vaccine for humans that would provide the same type of control," he said.

There is no cure for AIDS, but cocktails of drugs can keep the disease at bay for many years.

The human immunodeficiency virus that causes AIDS infects 33.3 million people globally, according to the United Nations agency UNAIDS. It has killed more than 25 million people.

Because it is spread in so many ways -- during sex, on needles shared by drug users, in breast milk and in blood -- there is no single easy way to prevent infection.

A vaccine is the best hope, and many drug companies and scientific research groups are working on various ways to try to develop one.

"The breakthrough here is in using a viral-delivered vaccine that persists -- essentially using an engineered virus to thwart a pathogenic virus," said Robin Shattock, a professor of mucosal infection and immunity at Britain's Imperial College, who was not involved in the research.

"Before this ... scientists had pretty much given up on the idea of a vaccine that could control HIV replication (but) this puts it firmly back on the agenda."

Efforts so far to make an AIDS vaccine have not been successful, but a 2009 study in Thailand involving 16,000 people showed for the first time that a vaccine could safely prevent HIV infection in a small number of volunteers.

Picker said the next step is to make a weaker version of the CMV virus to make sure it does not cause any problems in people.

"The concern would be if we move a virus that is not modified that in some small number it might cause disease," he said.

(Editing by Mohammad Zargham)

mochila insert follows