Scientists had hoped that the strain of HIV infecting the patients would prove harmless and so form the basis of a live vaccine that would be more effective than one based on "dead" HIV proteins.
Researchers around the world have reported that healthy people who have been infected with HIV for 12 years or more without any signs of ill-health - called long-term, non-progressors - have now developed the weakened immune systems that are the first signs of Aids.
One of the most important long-term survivors is an Australian man who has been infected for more than 17 years without developing Aids or taking medication until his immune system became suppressed earlier this year.
The man was a blood donor and had infected at least eight other people in the Sydney area before realising he was HIV positive. Each of the Sydney Blood Bank Cohort, as the group is known, is infected with a crippled form of HIV that lacks one of the virus's key genes, called nef.
The gene promotes the efficient production of the virus and helps it to evade the body's immune defences. For this reason it has become a prime target for scientists wanting to manipulate HIV genetically to produce a safe, live vaccine.
Three members of the cohort have died of causes probably unrelated to HIV but the other five, and the blood donor himself, were all healthy until recently and showed no signs of progressing to Aids.
In a study by Australian scientists, led by Jennifer Learmont of the Australian Red Cross Blood Service in Sydney, it was found that at least two of the blood recipients are now showing the early signs of Aids - indicated by a fall in the number of white blood cells.
Aids scientists were hoping that the crippled form of HIV would prove harmless enough for it to be used as the basis for a live, attenuated vaccine. Experience from other infectious diseases has shown these to be more effective than other types of vaccine.
Philippa Easterbrook, professor of HIV medicine at King's College London, said there are about 50 people in Britain who appear to be long-term, non-progressors.
But since they were enrolled in a national study of long-term HIV survivors, about half of them have begun to show the first signs of disease.
"The feeling now is that very few non-progressors are true non-progressors. They are just slow progressors. There is an attrition over time," Professor Easterbrook said.
The British group is part of an international effort to understand the natural protective factors that enable some people to live with HIV for a longer period of time than the average, she said.
Julian Meldrum, editor of the Body Positive newsletter, said the Sydney research has shown that it is now virtually inconceivable that a live vaccine could be based on HIV because of the long-term risks of developing Aids.
"What we now know is that we don't so much have long-term, non progressors but long-term, slow progressors who eventually get Aids," Mr Meldrum said.Reuse content