By Michael Kahn

LONDON (Reuters) - The AIDS virus is quickly adapting across large groups of people to avoid triggering the human immune system, posing another challenge in the search for a potential vaccine, researchers said on Wednesday.

Scientists know the human immunodeficiency virus, or HIV, constantly mutates within individual people to find ways to attack cells.

But the study published in the journal Nature suggests changes that help the virus do this are increasingly passed on in the wider population.

"What was previously clear is the virus could evolve within each infected person but that doesn't really matter from a vaccine perspective if the virus at the population level is staying the same," said Philip Goulder, an immunologist at Oxford University who led the study.

"The implication is that once we have found an effective vaccine, it would likely need to be changed to keep pace with the rapidly evolving virus."

There is no cure for AIDS and 33 million people globally are infected with HIV. Cocktails of drugs can control the virus and keep patients healthy. AIDS has killed more than 25 million people since the early 1980s, mostly in sub-Saharan Africa.

Researchers are trying to find vaccines that either prevent infection or would control the virus so that patients are less likely to transmit it -- a so-called therapeutic vaccine.

"The process of the virus adapting is happening before our eyes at quite a speed, and it is something we need to take into account when making our vaccines," Goulder said.

HIV attacks the immune system, the body's natural defenses. Like other viruses, it cannot replicate on its own but must hijack a cell and turn it into a virus factory.

HIV must evade several genes to do this, including an immunity gene called HLA.

The team, which included researchers from Australia and Japan, analyzed the genetic sequences of HIV and versions of HLA genes known to control the virus in 2,800 people.

Some people have a version of the gene that is more protective. In the study, the researchers found that mutations that allow HIV to evade immune responses directed by HLA were more common in people with the protective variant of the gene.

This was strong evidence for HIV adaptation to the human immune system among the wider population, Goulder added in a telephone interview.

This means the so-called escape mutation is circulating in more and more people and accumulating in the wider population of those infected with HIV, he said.

"We saw similar effects in every mutation that we looked at," Goulder said. "This shows that HIV is extremely adept at adapting to the immune responses in human populations that are most effective at containing the virus."

(Editing by Will Dunham and Jon Boyle)

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