The unique nature of HIV has hampered the search for an Aids vaccine and it remains a distant prospect, the world's leading experts say.
When American politicians announced the discovery of HIV in 1984, they predicted that a vaccine and a cure for Aids would be available within five years. It turned out to be a hopelessly optimistic assessment as the immense technical and scientific difficulties unfolded.
Nevertheless, the discovery of the virus led to important developments. The first was a blood test to determine whether someone was HIV positive. A global research effort into the genetics and biology of HIV led to a deeper understanding of the virus's modus operandi. This pointed to ways of sabotaging viral replication in infected patients.
In the mid-1990s, the first clinical trials with a combination of antiviral drugs demonstrated their immense potential as an effective form of treatment for HIV-positive people.
Professor Jonathan Weber of Imperial College said the dramatic impact of antiviral therapy transformed lives, but it did not solve the fundamental problem of the growing epidemic. "Antivirals have increased the number of people who are infected with HIV [by extending their lives] - but that's one of the reasons why we still need a vaccine," he said.
There are, however, inherent problems with developing a vaccine against HIV. The first is that it mutates rapidly, meaning that a vaccine developed against one form of the virus may not work against another. Another problem is that HIV integrates itself into the DNA of an infected person. A third difficulty is that the virus attacks the very cells of the immune system sent to attack it. Once infected, these T-cells take the virus deeper into the body's immune defences.
Albert Sabin, who developed the world's first oral polio vaccine, said in 1993 that he thought the problems posed by HIV would make it impossible to produce an effective Aids vaccine. He was proved right. The only putative Aids vaccine to reach the final of clinical trials was shown to be a dud in 2004.
In recent years, however, the focus of vaccine development has shifted. Instead of trying to produce a vaccine to prevent infection, scientists are hoping to develop a vaccine to prevent the onset of Aids in people who are HIV positive. These therapeutic vaccines are aimed at stimulating the disease-fighting cells of the immune system rather than its prophylactic antibodies. Merck, the US pharmaceuticals company, is leading the effort with a vaccine based on a genetically modified virus with three HIV genes. The US National Institutes of Health and a European consortium both have their own initiatives, with the help of government sources and private organisations such as the Bill and Melinda Gates Foundation.
However, few experts are predicting an Aids vaccine is around the corner. Andrew McMichael, professor of molecular medicine at Oxford University, who is leading Britain's vaccine effort, said: "I'm optimistic we'll get to it eventually, but I'm not optimistic about having a solution in three or four years."