Just over two dozen people near Lake Victoria have been found to remain uninfected even though they have unprotected sex with HIV-positive partners, a phenomenon termed "discordant couples".

Researchers found that the immune systems of the 28 resistant individuals behaved in surprising ways which, it is hoped, will point the way to a vaccine within 10 years.

Some of the resistant individuals had a lower measured immune response than infected partners but their immune systems attacked the virus more effectively, keeping them HIV negative. The finding suggests that what matters is quality, not quantity, of immune response.

The Ugandan results suggest resistant individuals are a more widespread and significant phenomenon than first realised, researchers said.

They expect to cause a stir by calling on the scientific community to focus half of vaccine research on resistant individuals, a dramatic scaling up of what has been until now a minority interest.

The research in Entebbe takes forward the findings from studies of a small group of commercial sex workers in Kenya. The Nairobi women's apparent immunity triggered a line of research which has led to the most promising vaccine now in trials, a joint enterprise between the universities of Oxford and Nairobi.

The findings come from the Uganda Virus Research Institute, which is backed by the International Aids Vaccine Initiative (IAVI), a not-for-profit organisation set up to channel funds into promising projects.

The Entebbe-based institute started phase one trials in February of the promising DNA-MVA vaccine designed by Pro fessor Andrew McMichael at Oxford University in collaboration with scientists in Nairobi. Specifically designed to combat the A strain of the HIV virus prevalent in east Africa, phase two trials are under way in the UK and Kenya.

The Ugandan discordant couple research is expected to be published this year.