AIDS has killed more than 25 million people worldwide [EPA]

An experimental vaccine has 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, US researchers have 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, US researchers said in a study published in the journal Nature on wednesday.

Doctor Louis Picker of the Oregon National Primate Research Center said he thinks it will be possible to have a vaccine ready to test in people within three years.

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

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.

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

Virus stopped

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.

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, there is no single easy way to prevent infection so a vaccine is the best hope, and many drug companies and scientific research groups are working on various ways to try to develop one.