The variations are located in genes that control signalling molecules called human leucocyte antigens (HLA), which help identify an intruder and tell the immune system to destroy it.

There are three types of HLA -- A, B and C.

The scientists took blood from 375 women with HIV in southern Africa who were not receiving anti-AIDS drugs, and analysed the samples to get their HLA profile.

Women who had one of two "protective" versions of HLA-B were likelier to survive and less likely to transmit the virus to their babies during pregnancy than counterparts who had one of two "deleterious" versions.

The study, published in the British journal Nature, adds to previous work that also points the finger at HLA-B variants.

That research, conducted among Caucasians, found that individuals with certain types of HLA-B genes had less virus in their bodies and a higher tally of CD4 immune cells, which are targeted by the human immunodeficiency virus (HIV).

"Differences in viral load and absolute CD4 count, and by inference, in HIV disease outcome, are principally related to HLA-B allele [gene variant] expression," the Nature study says.

The work is a joint project by the universities of Oxford, Harvard and KwaZulu-Natal, led by Oxford specialist Philip Goulder.

AIDS first came to notoriety in 1981, but the search for a vaccine has been deeply frustrating.

One reason is that no-one has ever been found to have the "natural correlates" for immunity against HIV.

If that individual exists and can be located, that would help vaccine engineers to mimick the workings of his or her immune system to design a protective formula.

Even though no-one has yet been found who is immune to HIV, there is sufficient variety in the human immune system to expose potential chinks in the virus' armour.

Some individuals can survive for years without having any of the symptoms of AIDS, whereas others very quickly develop the disease. Understanding and exploiting the genetic reasons for this big difference can help the search for a vaccine that may, at least, give partial protection.

Around 39.4 million people have HIV or AIDS, and the disease has killed more than 23 million people in its 23-year history, according to UN figures.

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