LONDON - People with a genetic variation that slows down HIV may also be causing a mutation to the AIDS virus that makes it less potent if transmitted to others, researchers said on Friday.

The human immunodeficiency virus that causes AIDS attacks immune system cells. 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.

"Some people have versions of the HLA gene that are known to force HIV to tolerate mutations that damage its ability to reproduce," Carolyn Williamson and Salim Abdool Karim at the Centre for the AIDS Program of Research in South Africa wrote in the Public Library of Science journal PLoS Pathogens.

The weaker virus causes slower disease progression in these people. Now it seems this weakened virus may get passed on and act the same way in others -- even if they do not have the protective HLA genes, Williamson told Reuters.

"The significant difference to other studies is that this is showing the actual benefit is due to the genetic composition of the virus," said Williamson, an AIDS researcher at the University of Cape Town who led the study.

"This study shows you can have a survival advantage with a virus containing specific genetic signatures associated with lower replication."

The South African study tracked 21 women without the beneficial form of HLA who were recently infected with the weakened strain of HIV. The researchers found the women had much lower levels of HIV in the body than those carrying a form of the virus which had not mutated in this way.

"It is pretty well established if you have certain HLA genes you are better off," Williamson said in a telephone interview. "It is very likely that the virus in the people who did not have the HLA genes came from individuals who did."

The researchers followed the women from between one to three years and found that while the levels of HIV in their bodies fell, their number of crucial CD4 T cells that coordinate the immune system rose.

The goal of treatment is to lower HIV levels to help the immune system renew itself and keep people healthier longer, in part to stem the spread of the virus.

The researchers have not yet studied the women to see how much slower they progressed to full-blown AIDS, but said the findings could help researchers in search of an effective vaccine by understanding better why some survive longer.

The AIDS virus infects an estimated 33 million people globally. It has killed 25 million people and there is no cure or vaccine.