A group of prostitutes thought to be immune to HIV have now become infected, causing dismay to scientists hoping to develop an Aids vaccine.

It was thought that exposure to HIV on a regular basis created immunity, but six Kenyan women previously thought to be resistant are now HIV-positive.

The women are all former prostitutes who have since left the business.

Scientists now think immunity may be reliant on continued exposure - once regular contact with HIV stops, immunity is lost.

Researchers at Oxford University have been monitoring 43 Kenyan prostitutes constantly exposed to HIV who had not been infected.



This implies that to maintain immunity, you need to have continual exposure

Sarah Rowland-Jones

Sarah Rowland-Jones, a researcher at Oxford University, described the development of HIV in the six women as "dismaying". She added: "This implies that to maintain immunity, you need to have continual exposure."

The final results have not yet been compiled, however, and the Medical Research Council, under which the research is being carried out, does not believe the chances of developing an effective vaccine have been damaged.

Hope

It is still hoped that a vaccine can be developed which builds up immunity by introducing genes containing fragments of HIV proteins into the body.

The Kenyan women studied by the Oxford scientists have large numbers of white blood cells known as cytotoxic T lymphocytes (CTLs), which are primed to kill other cells in which HIV hides.

The scientists devised a vaccine using this information, consisting of a small loop of DNA, containing fragments from genes for three different HIV proteins. It also contains a live, disabled cowpox virus, used in small pox vaccines, with the same gene fragments added to its genome.

The vaccine effectively stimulated production of CTLs in monkeys and protected against HIV infection.

But the development of HIV in the six Kenyan women may mean that to be effective, the vaccine would need to be given repeatedly, making it too expensive for the developing world which is most affected by the Aids epidemic.

The International Aids Vaccine Initiative, which is sponsoring the vaccine's development, is not abandoning the research, reports New Scientist magazine.

Combined

Scientific adviser to the initiative Jaap Goudsmit, of the University of Amsterdam, believes the Oxford vaccine could be combined with other components.

He said: "We want as many approaches as possible for phase one studies and then we can choose between them or mix and match."

Safety trials of the Oxford vaccine will go ahead this year with 30 British volunteers, to be followed by further trials in Kenya if they go well.

Gavin Hart at the National Aids Trust, said the development of HIV in the women was "disappointing".

But he added: "It is still a very important path of vaccine research that we have to continue to follow. There is still a lot of potential to this path."

It was important to continue to support the Oxford research, he said.