More than 13 million children worldwide have been orphaned by AIDS

Researchers have discovered that a small group of Ugandans seem to have natural protection against HIV.

The two dozen or so individuals have not contracted the virus despite having unprotected sex with partners who are HIV positive.

Studying how their immune systems keep the virus at bay could help scientists develop an effective vaccine against Aids.

According to a consensus of scientists, vaccinating against Aids should be possible one day. Attempts so far have not proved successful but trial vaccines are being tested around the world.

Clues to combating the disease, which has killed more than 25 million men, women and children worldwide, come from a minority of individuals who are repeatedly exposed to HIV but never succumb to infection.

The first example of this was found in Kenya among sex workers who appear immune to HIV.

Follow-up research by the universities of Oxford and Nairobi has led to a prototype vaccine which is being trialled in the UK and Kenya.

Slow process

The latest findings relate to a small group of people living near Lake Victoria in Uganda.

They appear to be able to stay HIV negative because their immune systems are able to fight HIV.

The research, by the Uganda Virus Research Institute, is expected to be published later this year.

Professor Andrew McMichael of Oxford University, UK, a leading Aids vaccine researcher, said the work was encouraging.

"It strengthens the case that the immune response can protect against infection so if you can make that immune response with a vaccine you can protect people," he said.

Professor McMichael developed the trial DNA-MVA vaccine in collaboration with scientists in Nairobi.

He said the Ugandan research will help them evaluate the existing vaccine and perhaps provide clues to how to modify it to improve it.

"It's a painfully slow process to develop a vaccine," he told BBC News Online.

"Any increased evidence that this is the right approach is very helpful."