A vaccine has cut the risk of infection with HIV, the virus that causes Aids, for the first time in a breakthrough hailed as a "historic milestone" in the global fight against the epidemic.
The experimental drug cut the risk of becoming infected with HIV by more than 31 per cent in the world's largest Aids trial of more than 16,000 volunteers in Thailand, researchers have announced.
It is the first time in human trials that a vaccine has stopped the virus, which infects 7,500 worldwide every day.
Dr. Anthony Fauci, director of the United States National Institute of Allergy and Infectious Diseases, warned the development was "not the end of the road," but said he was surprised and very pleased by the outcome.
"It gives me cautious optimism about the possibility of improving this result" and developing a more effective Aids vaccine, he said. "This is something that we can do."
"Today marks a historic milestone," said Mitchell Warren, executive director of the Aids Vaccine Advocacy Coalition, an international group that has worked toward developing a vaccine.
"It will take time and resources to fully analyse and understand the data, but there is little doubt that this finding will energise and redirect the Aids vaccine field.
Even a partially effective vaccine could have a big impact. In 2007, two million died of Aids according to the United Nations agency UNAIDS.
Colonel Jerome Kim, who helped lead the study for the US Army, which was also involved in the trial, said: "It is the first evidence that we could have a safe and effective preventive vaccine."
The Thailand Ministry of Public Health conducted the study, which used strains of HIV common in Thailand. Scientists stressed it is not clear whether the vaccine would work against other strains in the United States, Africa or elsewhere.
The study tested a two-vaccine combination in a "prime-boost" approach, where the first injection primes the immune system to attack HIV and the second strengthens the response.
The vaccines are ALVAC, from Sanofi Pasteur, the vaccine division of French drugmaker Sanofi-Aventis; and AIDSVAX, originally developed by VaxGen Inc. and now held by Global Solutions for Infectious Diseases, a non-profit founded by some former VaxGen employees.
ALVAC uses canarypox, a bird virus altered so it can't cause human disease, to ferry synthetic versions of three HIV genes into the body. AIDSVAX contains a genetically engineered version of a protein on HIV's surface. The vaccines are not made from whole virus — dead or alive — and cannot cause HIV.
Neither vaccine in the study prevented HIV infection when tested individually in earlier trials, and dozens of scientists had called the new one futile when it began in 2003.
"I really didn't have high hopes at all that we would see a positive result," Dr Fauci confessed.
The study tested the combination in HIV-negative Thai men and women ages 18 to 30 at average risk of becoming infected. Half received four "priming" doses of ALVAC and two "boost" doses of AIDSVAX over six months. The others received dummy shots.
All were given condoms, counselling and treatment for any sexually transmitted infections, and were tested every six months for HIV. Any who became infected were given free treatment with antiviral medicines.
Participants were followed for three years after vaccination ended.
The results were that new infections occurred in 51 of the 8,197 given vaccine and in 74 of the 8,198 who received dummy shots. That worked out to a 31 per cent lower risk of infection for the vaccine group.
The vaccine had no effect on levels of HIV in the blood of those who did become infected, providing "one of the most important and intriguing findings" of the trial, according to Dr Fauci, giving scientists important clues in identifying whether treatment drugs are actually make a difference by giving protection to the immune system.
Full details of the $105 million study will be given at a vaccine conference in Paris in October.
This is the third big vaccine trial since 1983, when HIV was identified as the cause of Aids. In 2007, Merck & Co. stopped a study of its experimental vaccine after seeing it did not prevent HIV infection. Later analysis suggested the vaccine might even raise the risk of infection in certain men. The vaccine itself did not cause infection.
In 2003, AIDSVAX failed in two large trials — the first late-stage tests of any Aids vaccine at the time.
It is unclear whether vaccine makers will seek to license the two-vaccine combination in Thailand. Before the trial began, the United States Food and Drug Administration said other studies would be needed before the vaccine could be considered for public use.
Also unclear is whether Thai volunteers who received dummy shots will now be offered the vaccine. Researchers had said they would do so if the vaccine showed clear benefit — defined as reducing the risk of infection by at least 50 per cent.
The study was done in Thailand because US Army scientists did pivotal research in that country when the Aids epidemic emerged there, isolating virus strains and providing genetic information on them to vaccine makers. The Thai government also strongly supported the idea of doing the study.