Scientists say they have developed a vaccine that cuts the risk of HIV infection by more than 30%.
It is the first time a vaccine has been shown to give even this limited protection against the virus that causes Aids.
The vaccine trial, which was funded by the US army, involved a combination of two vaccines that individually had proved ineffective.
The World Health Organisation says it offers the promise of a safe vaccine eventually becoming available for people around the world.
The BBC's correspondent in Bangkok, Alastair Leithead, spoke to Dr Joseph Chiu, the representative of the US army surgeon general, who has been involved in the work.
Dr Saladin Osmanov is the co-ordinator for the WHO Aids Initiative Vaccine Research.
Yolishwa Notshe is an HIV/Aids co-ordinator with the charity Action Aid in South Africa, which has one of the highest HIV infection rates in the world.
The BBC's Dan Damon spoke to both of them about this new development, Dr Saladin Osmanov first.
First broadcast 24 September 2009
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