The vaccine – a combination of two earlier experimental vaccines – was given to 16,000 people in Thailand, in the largest ever such vaccine trial. Researchers found that it reduced by nearly a third the risk of contracting HIV, the virus that leads to Aids.
It has been hailed as a significant, scientific breakthrough, but a global vaccine is still some way off.
The study was carried out by the US army and the Thai government over seven years on volunteers – all HIV-negative men and women aged between 18 and 30 – in some of Thailand's most badly-affected regions.
The vaccine was a combination of two older vaccines that on their own had not cut infection rates. Half of the volunteers were given the vaccine, while the other half were given a placebo – and all were given counselling on HIV/Aids prevention.
Participants were tested for HIV infection every six months for three years.
The results found that the chances of catching HIV were 31.2% less for those who had taken the vaccine – with 74 people who did not get the vaccine infected and 51 of the vaccinated group infected.
Some 33 million people around the world have HIV – with Rwanda alone having some 290,000 people infected with the virus.
With additional files from BBC and agencies