The vaccine reduced the chance of being infected by a third, researchers announced after the world's largest trial of 16,000 volunteers, carried out by the US Army and Thailand's Ministry of Public Health.

The surprising result comes after years of fruitless attempts by the medical world to find an HIV vaccine, including one trial jab that apparently boosted infection rates.

"It is the first demonstration that a vaccine against HIV can protect against infection," Colonel Jerome Kim of the US military HIV research programme told a news conference in Bangkok via videolink.

"This is a very important scientific advance and gives us hope that a globally effective vaccine may be possible in the future," he said.

Thai Public Health Minister Witthaya Kaewparadai said the "outcome of this study is a scientific breakthrough."

The vaccine was a combination of two older drugs that had not reduced infection on their own, and the researchers said they were now studying why the two apparently worked together.

The study combined the canarypox vaccine ALVAC, manufactured by Sanofi-Aventis of France, and AIDSVAX, originally made by VaxGen Inc and now licensed to the non-profit Global Solutions for Infectious Diseases.

Researchers said the latest vaccine showed a 31.2 percent efficacy in reducing the risk of HIV infection.

"The outcome represents a breakthrough in HIV vaccine development because for the first time ever there is evidence that HIV vaccine has preventative efficacy," said the research team in a statement.

The vaccine was tested on volunteers -- all HIV negative men and women aged from 18 to 30 -- at average risk of infection in two Thai provinces near Bangkok starting in October 2003.

Half received the vaccine and the rest were given a placebo. Out of the placebo recipients 74 of 8,198 became infected compared with 51 of 8,197 who got the vaccine.

The World Health Organization and the Joint UN Programme on HIV/AIDS congratulated the researchers for the "encouraging" results.

"The study results, representing a significant scientific advance, are the first demonstration that a vaccine can prevent HIV infection in a general adult population and are of great importance," they said in a statement.

The UN said it may not be possible to get licensing permission for the drug at the moment based on the results, and that further studies were needed to determine if the vaccine has the same effect in other parts of the world.

AIDS first came to public notice in 1981 and has since killed at least 25 million people worldwide, and 33 million others are living with AIDS or the HIV virus.

Swift progress in identifying the virus that caused AIDS unleashed early optimism that a vaccine would quickly emerge. HIV destroys immune cells and exposes the body to opportunistic disease.

But out of the 50 candidates that have been evaluated among humans, only two vaccines have made it through all three phases of trials, and both were flops. About 30 vaccines remain in the pipeline.

Scientists were in 2007 forced to abandon two advanced clinical trials of a vaccine by pharmaceutical company Merck after it appeared to actually heighten the risk of AIDS infection.

Sanofi Pasteur, the vaccines division of Sanofi-Aventis, said the results of the latest test, although "modest", were the first concrete demonstration that a vaccine "could one day become a reality."

The Global HIV Vaccine Enterprise, an alliance of researchers, policymakers, donors and advocates that includes the Bill and Melinda Gates Foundation, said it was a "historic day in the 26-year quest to develop an AIDS vaccine."

The International AIDS Vaccine Initiative (IAVI), an organisation that promotes the search for a vaccine, said the trial results were "very exciting and a significant scientific achievement."

But a top AIDS scientist, France's Jean-Francois Delfraissy, warned that the results were "good news but the effect remains modest".

One expert said the treasure would revolutionise understanding of the Anglo-Saxons, a Germanic people who ruled England from the fifth century until the Norman conquest in 1066. Another said the find would rank among Britain's best-known historic treasures.

"This is just a fantastic find completely out of the blue," Roger Bland, who managed the cache's excavation, told The Associated Press. "It will make us rethink the Dark Ages."

Bland said the hoard was unearthed in what was once Mercia, one of five main Anglo-Saxon kingdoms, and is thought to date to 675-725 AD.

A total of 1,345 items have been examined by experts and 56 lumps of earth were found to contain metal artefacts detected by an X-ray machine, meaning the total will likely rise to about 1,500.

"I think wealth of this kind must have belonged to a king but we cannot say that for absolute certain," Bland said.

The Anglo-Saxons were a group of Germanic tribes who gradually invaded England by sea starting in the fifth century in the wake of the collapse of the Roman Empire. Originally, they came from what is now the coastal region of northwest Germany.

Their artisans made striking objects out of gold and enamel and they also created poetry that still amazes people today. Their best-known literary work is Beowulf, an anonymous epic poem about a warrior who does battle with monsters and a dragon.

Their language, Old English, is a distant precursor of modern English. It supplies many of the structurally important words such as pronouns and prepositions as well as words for everyday concepts.

Unfortunately, much of their literature and artwork have been lost through warfare, looting, upheavals and the passage of time. Scholars must deduce what their culture was like using often scanty evidence.

Leslie Webster, the former curator of Anglo-Saxon archaeology at the British Museum, said the amount of gold uncovered - about 5kg - suggested that early medieval England was a far wealthier place than previously believed.

She also said the crosses and other religious artefacts mixed in with the mainly military items, such as sword pommel caps, might shed new light on the relationship between Christianity and warfare among the Anglo-Saxons.

The seventh-century hoard found by 55-year-old Terry Herbert in western England, consists of at least 650 items of gold and 530 silver objects, along with some copper alloy, garnets and glass.

Most of the objects are ornaments for weapons and other military artefacts, some inlaid with precious stones.

Herbert, from the town of Burntwood, found the gold on a friend's farm on July 5 and spent the next five days scouring the field for the rest of the hoard.

Herbert recovered the first items before professional archaeologists took over the excavation.

"Imagine you're at home and somebody keeps putting money through your letterbox, that was what it was like," Herbert said. "I was going to bed and in my sleep I was seeing gold items."

One of the most intriguing objects is a small strip of gold inscribed with a warlike Latin quotation from the Old Testament, which translates as: "Rise up, O Lord, and may thy enemies be dispersed and those who hate thee be driven from thy face."

Most of the other treasures unearthed in the hoard appear to be military-related, and the strip may have been fastened to a shield or a sword belt.

The hoard was officially declared treasure by a coroner, which means it will now be valued by a committee of experts and offered up for sale to a museum. Proceeds would be split 50-50 between Herbert and his farmer friend, who has not been identified. The find's exact location is being kept secret to deter looters.

Bland said he could not give a precise figure for the worth of the hoard, but he said the treasure hunter could be in line for a "seven-figure sum".

Herbert said the experience had been "more fun than winning the lottery," adding that one expert likened his discovery to finding Tutankhamen's tomb.

"I just flushed all over when he said that. The hairs on the back of my neck stood up," Herbert said.

The hoard is in storage at the Birmingham Museum and Art Gallery. Some of the items are due to go on display starting on Friday.

"The quantity of gold is amazing but, more importantly, the craftsmanship is consummate," said archaeologist Kevin Leahy, who catalogued the find. "This was the very best that the Anglo-Saxon metalworkers could do, and they were very good."

Leahy said there was still much to learn about the treasure, its purpose, and its origins.

"It looks like a collection of trophies, but it is impossible to say if the hoard was the spoils from a single battle or a long and highly successful military career," he said. "We also cannot say who the original, or the final, owners were, who took it from them, why they buried it or when. It will be debated for decades."

Bland agreed, saying that archaeologists were still baffled by the function of many of the pieces they found.

"There's lots of mystery in it," he said.

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