A commercially developed vaccine has successfully protected mice and ferrets against the highly pathogenic H5N1 avian influenza virus, U.S. scientists reported on Tuesday.

The vaccine, developed by Vical Incorporated in San Diego, California, is also effective against multiple human and bird influenza strains, according to researchers at the St. Jude Children's Research Hospital in Memphis, Tennessee, where the animal test was conducted.

This finding suggests that such a vaccine would protect humans against multiple variants of the bird and human influenza viruses, said Richard Webby, the doctor who led the study.

Such a vaccine could protect humans against an H5N1 "bird flu" virus that mutates so that it adapts to humans and can readily spread from person to person, Webby said. Flu experts and public health officials fear that such an H5N1 variant would trigger a worldwide human flu epidemic.

In the study, the investigators used two versions of Vical's DNA-based vaccine. One vaccine was directed against three viral proteins: the nucleoprotein (NP), the matrix protein (M2) and hemagglutinin (H5). The other version of the vaccine contained only the NP and the M2.

The NP and the M2 proteins are "conserved" proteins that generally do not mutate quickly, and therefore are slow to avoid immune responses triggered by the vaccine.

The H5 is a highly variable protein on the surface of the flu virus that is critical to their ability to infect cells. This variable protein is known to mutate readily, thereby foiling previous immune responses it triggered -- whether due to natural exposure or vaccination.

The study included 16 mice or six ferrets in each vaccine or control group. According to Webby, the full, three-component vaccine provided complete protection in mice against lethal challenges with a highly virulent H5N1 virus strain (Vietnam/1203/2004).

The second vaccine also provided significant protection against several strains of human influenza virus as well as the H5N1 "birdflu" strain, said the researchers. Fourteen of the 16 mice in each of two vaccine groups survived with moderate weight loss.

"Such cross-protection against bird and human influenza is considered by researchers to be the 'Holy Grail' of flu vaccines, "Webby said in a statement.

"By stimulating immune responses against targets not likely to mutate, the vaccine could trigger an immune defense against a broad range of variants of the virus."

If a pandemic flu strain is emerging, this approach may provide broad-based protection prior to the identification and development of a precisely matched hemagglutinin vaccine component, noted the researchers.

"Even if the bird flu virus mutates so it becomes adapted to humans, this kind of cross protection will allow the immune system to track and attack such an emerging new variant without missing a beat," Webby said.

"We wouldn't have to wait to start developing a vaccine against it until after the original virus mutated."

"We are encouraged by the protection afforded by our avian flu vaccine and by the cross-protection data," said Vijay B. Samant, president and chief executive officer of Vical, adding the company is "advancing toward human trial of the flu vaccine."