An experimental HIV vaccine has for the first time cut the risk of infection, researchers say.
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 parts of Thailand.
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.
The vaccine is based on B and E strains of HIV that most commonly circulate in Thailand not the C strain which predominates in Africa.
"This result is tantalisingly encouraging. The numbers are small and the difference may have been due to chance, but this finding is the first positive news in the Aids vaccine field for a decade," said Dr Richard Horton, editor of the Lancet medical journal.
"We should be cautious, but hopeful. The discovery needs urgent replication and investigation."
Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases said: "For the first time, an investigational HIV vaccine has demonstrated some ability to prevent HIV infection among vaccinated individuals.
"Additional research is needed to better understand how this vaccine regimen reduced the risk of HIV infection, but this is certainly an encouraging advance for the HIV vaccine field."
The findings were hailed by the World Health Organization (WHO) and the Joint United Nations Programme on HIV/Aids (UN/Aids).
They said while the results were "characterised as modestly protective... [they] have instilled new hope in the HIV vaccine research field".
Some 33 million people around the world have HIV.