Two doses of the vaccine produced an immune system response potent enough to neutralize the virus in tests on 113 volunteers who were injected as part of a federally sponsored study being conducted at three US universities.

``This is very good news,'' said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. ``This is the first vaccine that anybody has that has been tested to show that you can actually produce a robust immune response.''

Public health authorities are alarmed by the H5N1 virus, which has been spreading primarily in birds across Asia and in Russia. It has also infected nearly 100 humans in the past 18 months, killing about half of them. If the virus starts to spread efficiently among humans, experts fear it could trigger a global pandemic that could kill millions.

In response, millions of birds throughout Asia have been slaughtered to try to stem the spread of the virus; governments and the World Health Organization have been stockpiling antiviral drugs, and scientists have been scrambling to produce an effective vaccine.

Much more testing will be needed to determine how the vaccine could be used, and other hurdles remain, including being able to produce and distribute large quantities of vaccine in the event of a pandemic, said Fauci. But he said the results represent a crucial milestone.

``The data are pretty solid,'' he said. ``They're preliminary, but solid. Having a vaccine in case we had a pandemic flu is a very important part of a comprehensive response to a pandemic.''

Other experts agreed that the tests marked a key step in the world's attempt to prepare for a possible pandemic.

``These are very important studies,'' said the University of Minnesota's Michael Osterholm. ``We all had anticipated that two doses were going to work. If it didn't, we would be in trouble.''

But he said the world will still be woefully unprepared if a pandemic occurs. The capability to produce and distribute a large amount of vaccine quickly, for example, is lacking.

``That's the Achilles' heel of our public health response,'' he said. ``These data are encouraging and supportive, but in the end a lot of people would be left unvaccinated in the event of a pandemic.''

The new vaccine results stem from a study being conducted at the University of Maryland in Baltimore, the University of Rochester in New York and the University of California, Los Angeles. Researchers tested a vaccine made by Sanofi-Pasteur, a French company, using a version of the virus originally isolated in Vietnam that has had a gene removed to make it harmless.

The vaccine was administered in four dosages to 452 healthy adults beginning in April. They received a booster shot four weeks later.

Blood samples showed the level of antibodies produced by the subjects' immune systems rose in direct proportion to the dose they received, with the strongest dose producing what is considered an adequate response, said Fauci.

The US government has already purchased two million doses and plans to buy more. The results will help officials determine how to package the vaccine for use if necessary.

``These results will guide us to the appropriate dose and dose regimen,'' said Fauci.

After further studies in healthy adults, including testing another booster shot, researchers next plan to begin evaluations on other groups, such as the elderly and children - two groups that tend to be particularly vulnerable to the flu, said Fauci.