Are Some People Immune to the AIDS Virus?

Is everybody who gets infected with the AIDS virus doomed to progress to full-blown AIDS? As it turns out, the answer is no. Some people carry the virus for years without experiencing any ill effects; the author of one recent study suggests that they never will. Two weeks ago, European researchers announced that some babies born with HIV (an inheritance from their HIV-positive mothers) appear to be able to shake the virus out of their systems. Some people even seem to be totally immune to infection despite repeated, prolonged exposure to HIV.

(See TIME’s photo-essay “Access to Life.”)

AIDS researchers are focusing on these rare cases, hoping that somewhere in these people — or in the strain of the virus they are carrying — lie clues to better treatments, an effective vaccine or even, someday, a cure. Scientists now estimate that perhaps 8% to 10% of those infected with HIV are what are called long-term nonprogressors — people who have not suffered any apparent damage to their immune system in at least a decade. And around 6% of those diagnosed with clinical AIDS may be considered long-term survivors (living five years or more).

The strongest evidence that some people are immune to viral infection comes from Africa, where scientists are studying 58 Nairobi women who have worked as prostitutes. None of them show a trace of HIV in their systems, though each has had unprotected sex with hundreds, perhaps thousands, of HIV-infected men. Scientists suspect their blood may be blessed with killer T cells that are particularly ferocious, able to totally annihilate the invading virus before it can take hold and replicate.

(See pictures of Africa’s AIDS crisis.)

In Sydney, Australia, meanwhile, researchers have been closely following the medical histories of seven people who became infected as a result of transfusions with HIV-tainted blood 15 years ago. None of them have become ill with AIDS; neither, it turns out, has the donor. Scientists have speculated that the virus that infected them is missing snippets of key genetic material that hampers its ability to reproduce — while at the same time protecting the carriers from infection by more robust HIV strains.

The AIDS-resistant babies, by contrast, remain baffling. A newborn’s immune system isn’t fully functional until around 18 months, so how could it successfully fight off an HIV invasion? But a report published in Lancet suggests that’s just what the infant immune systems did. In the study, researchers followed more than 250 children in Sweden, Belgium and Italy, testing them at intervals for the presence of both antibodies to HIV and the virus itself.

In nine cases, babies who were initially positive on both tests became antibody negative within months. Researchers subsequently found no trace of the virus in six of those youngsters. The results indicate that the children have either cleared the AIDS virus from their systems or developed a permanent tolerance for it.

— Reported by Helen Gibson / London and Lawrence Mondi / New York

See TIME’s special report “How to Live 100 Years.”

See the top 10 medical breakthroughs of 2009.