In clinical trials, the new drug not only required fewer scarce antigens but also triggered a far more effective immune response, according to a report published in British journal The Lancet on Friday.

The vaccine - which combines a standard vaccine with a patented oil-in- water emulsion - produced up to six times as many antibodies capable of naturalizing the bird flu virus as the same dose without the emulsion.

Even in weaker doses, the drug from GlaxoSmithKline - which funded the study - was highly effective.

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"The number of pandemic vaccine doses can be stretched 20-25 fold," said virologists Suryaprakash Sambhara of the US Centers for Disease Control and Prevention in Atlanta and Gregory Poland of the May Vaccine Research Group in Rochester, Minnesota.

The highly pathogenic H5N1 strain has caused widespread outbreaks in birds - especially in Asia - as well as lethal infections in humans. The human body does a poor job of producing antibodies to defend against the virus, especially haemagglutinin H5.

Of 318 laboratory-confirmed cases reported by the World Health Organization, 192 have been fatal.

Scientists fear the virus strain could mutate and move easily among humans. To date, almost all of those who contracted the disease were in close contact with infected poultry.

The world's drugmakers are already unable to meet demand for a standard seasonal flu vaccine. In a pandemic such as the Spanish flu, which left up to 50 million dead nearly a century ago, it would be impossible under current conditions to produce enough vaccines.

Moreover, because most people have never been exposed to H5N1 - making them "immunologically naive" in the jargon of virologists - a single dose of vaccine would probably prove inadequate.

In the study, four antigen doses ranging from 3.8 micrograms to 30 micrograms were given to 400 adults, aged 18-60, divided into eight groups of 50.

Four groups received doses with the emulsion, and four groups without.

Even at the lowest dose of 3.8 micrograms, the adjuvanted drugs produced up to twice as many antibodies as the 30 microgram doses without the added solution. AGENCE FRANCE-PRESSE