An experimental vaccine developed in the United States to protect people against a lethal strain of bird-flu virus has proved effective in laboratory tests.

Dr. Anthony Fauci,  a noted American government expert on infectious diseases, says the vaccine produced a strong immune response to bird flu in a significant percentage of volunteers, 113 of 452 people.

Scientists are conducting further tests, giving vaccine to people over the age of 65 and to children, but even now, Dr. Fauci says the preliminary results are very important. The first tests indicate the vaccine can protect human populations if the current bird-flu outbreak in Asia becomes a global flu pandemic, with potentially catastrophic consequences.

Scientists have been racing to develop an effective vaccine against the H5N1 strain of bird-flu virus that has been spreading through birds in Asia and parts of Russia for nearly two years. A number of people, who had close contact with infected birds, have been infected, and more than 60 have died, a fatality rate of roughly 50 percent, but the virus has not yet changed into a form that can spread directly from one human to another.

Scientists say this would trigger a global pandemic with potentially catastrophic consequences. Asked about the risk of virus mutation, Dr. Fauci has told VOA, "The circumstances are such that invariably, sooner or later, it's going to happen."