LOS ANGELES, March 6 (Xinhuanet)-- Experts are developing new vaccines against the new H5N1 bird flu strain that is spreading in the world, a top U.S. health official said on Monday.

    According to Mike Leavitt, U.S. Health and Human Services Secretary, the virus strain that causes bird flu outbreaks in Africa, Europe, and the Middle East this year is different from an earlier version.

    The virus has mutated, Leavitt said, and it will mutate further.

    Experts feared it could become a deadly human flu that spreads easily around the globe with the potential to kill millions.

    "We need to continue to develop new vaccines," Leavitt told an immunization conference in Atlanta.

    U.S. laboratories developed a vaccine based on a H5N1 virus strain obtained in Vietnam in year 2004, but researchers note the emergence of a second version of bird flu, an Indonesian strain.

    Influenza viruses mutate constantly, allowing them to find new ways to infect animals and humans. Now U.S. researchers plan to create a new vaccine targeted at the second variety called A/ Indonesia/5/2005, Leavitt said.

    The second vaccine may give drugmakers a head start if a version of the virus similar to the Indonesia strain begins spreading in people, added Anthony Fauci, director of U.S. National Institute of Allergy and Infectious Diseases.

    Using a prototype virus developed by the Centers for Disease Control and Prevention, drug companies can determine how best to grow proteins that will be useful in a pandemic strain of the vaccine, Fauci said.

    Health officials are trying to encourage increased development and use of flu vaccines. Several pharmaceutical firms have won contracts to make vaccines against potential pandemic flu strains.

    The U.S. Food and Drug Administration gave vaccine makers a blueprint for gaining approval for pandemic vaccines on March 2, the first time the agency had laid out guidelines for flu vaccine clearance. Enditem