"It is the first demonstration that a vaccine against HIV can protect against infection."

The vaccine was effectively a combination of two older vaccines that had not cut infection on their own.

'Cautious optimism'

Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said he was pleased by the results but cautioned that this was "not the end of the road".

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"It gives me cautious optimism about the possibility of improving this result," he said.

Although the vaccine only proved effective in 31 per cent of cases, such a cut in the rate of infection could have a dramatic impact on the spread of the disease.

About 7,500 people worldwide are newly infected with HIV every day and two million people died of Aids in 2007, according to the UNAids agency. An estimated 25 million people have died since the virus was discovered in the 1980s.

"Today marks an historic milestone," Mitchell Warren, the executive director of the AIDS Vaccine Advocacy Coalition, an international group that has worked toward develping 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."

Thai strain

But scientists also cautioned that the vaccine had only been tested on strains of HIV common in Thailand. Whether such a vaccine would work against other strains in the US, Africa or elsewhere in the world is unknown.

The vaccine was also found to have no effect on the virus once it is in the body. Patients who became infected had the same amount of the virus in their blood and a similar amount of damage to their immune systems despite having the vaccine.

"Further research will be required so that the products can be developed and later on licenced," Saladin Osmanov, a co-ordinator with the World Health Organisation and the UN's programme for HIV/Aids vaccine initiative, said.

"But the important thing to say is that even when the vaccine is available, given the low level of efficacy ... these types of vaccines will have to be used as a complimentary tool with other prevention programmes," he told Al Jazeera.

Before the trial began, the US Food and Drug Administration said other studies would be needed before the vaccine could be considered for US licensing.