WASHINGTON, April 17 (Reuters) - An experimental bird flu vaccine that uses a common cold virus and bits of DNA from the H5N1 virus appears to stimulate an immune response in mice, U.S. researchers said on Thursday.

They said their experiment is a first step towards developing a next-generation bird flu vaccine that does not need to be grown for months in chicken eggs and that could protect against mutated versions of the virus.

"We want to have a vaccine that can be stored in advance and have the potential to provide protection for a period of time until we can change the vaccine to match the latest form of avian influenza," said Suresh Mittal of Purdue University in Indiana, who worked on the study.

"The combination of flu genes that we've used to produce the vaccine, I think, will provide that capability."

The H5N1 avian influenza virus currently mostly affects birds and is sweeping though flocks in many parts of Asia, Africa and occasionally in Europe.

It can rarely pass to humans and has infected 381 people since 2003, killing 240 of them, according to the World Health Organization.

At least 16 companies are working on vaccines to prevent bird flu infection in people, but the process is problematic. Flu vaccines are hard to make because they must be grown in chicken eggs for months, and the viruses themselves mutate every year.

The seasonal flu vaccine must be reformulated every year and no one knows what would happen if H5N1 mutated into a form that people could transmit easily to one another.

If a pandemic broke out, using current technology it would be close to a year before anyone could be vaccinated.

Mittal, Mary Hoelscher of the U.S. Centers for Disease Control and Prevention and colleagues worked with H5N1 virus samples from Vietnam and Indonesia to make a vaccine that they hoped would work against even "drifted," or mutated, strains.

They used a common cold virus, known as an adenovirus, to carry H5N1's hemagglutinin gene, which give flu strains the "H" of their names.

Most current flu vaccines also focus on hemagglutinin.

They also used another gene called nucleoprotein or NP, which has not been used in flu vaccines. The hope is that the NP gene will both promote an immune response against flu, and perhaps be a bit more stable than the highly mutation-prone hemagglutinin.

So far the researchers have only tested mice, but the vaccine caused a strong immune response that lasted at least a year.

"In humans we want a vaccine to be fully effective for at least a year," Mittal said in a statement.

"This approach may prevent severe illness and death or shorten the course of future infection with H5N1 virus strains that are antigenically distinct from currently circulating strains, and it may offer stockpiling advantages that overcome the limitations associated with storage of egg-derived vaccines," the researchers wrote.

Their technology has been licensed to PaxVax Inc, a privately held San Diego-based corporation. (Reporting by Maggie Fox; Editing by Will Dunham and Eric Walsh)

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