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Researchers at the U.S. Centers for Disease Control have created a prototype of an avian flu vaccine that could hold significant promise for use in an influenza pandemic.

The vaccine — made by engineering a human cold virus to produce a protein made by the H5N1 flu virus — would likely be quicker to make than standard flu vaccine, induce a better immune response and could be made without reliance on eggs, the researchers said in a study rushed to print by The Lancet, a prominent medical journal.

Influenza vaccine experts unrelated to the work praised the study, but noted as yet the vaccine has only been tested in mice.

“It's very, very encouraging,” said Dr. Robert Belshe, director of the Center for Vaccine Development at Saint Louis University School of Medicine in Missouri.

“(But) I've got a sign over my door that says: You can prove anything in mice. And so we need to get a clinical trial in humans and look at that information. And that would really be critically important.”

The CDC team, led by Canadian scientist Suryaprakash Sambhara, genetically modified an adenovirus — one of the causes of common colds — to produce the protein made by the hemagglutinin gene on the surface of the H5N1 virus.

The adenovirus was attenuated or altered so that it would not cause disease, making it a delivery system to introduce the immune system to the H5N1 hemagglutinin. That is the key feature of the avian flu virus that the immune system would need to recognize in order to mount an effective immune response.

Mice inoculated with two doses of the adenovirus-based vaccine were protected when they were later exposed or challenged by H5N1 viruses.

Researchers at the University of Pittsburgh have also designed an H5N1 vaccine using an adenovirus as the vector or delivery system. They reported on their work in a study published last week by the Journal of Virology.

But an interesting twist to the CDC research relates to the H5N1 virus that provided the hemagglutinin for their vaccine. It was taken from a virus that circulated in 1997. There has been considerable mutation or what's called antigenic drift in the hemagglutinin in the intervening years.

Flu viruses are constantly drifting in this way; it's why getting a flu shot one year won't protect you against next year's circulating flu strains.

And yet, when the mice were challenged with viruses isolated in Vietnam in 2003 and 2004, the 1997 vaccine was protective.

That suggests this type of vaccine is more cross protective than regular flu vaccines, creating the possibility that pandemic vaccine could be made and stockpiled in advance, the authors said.