The team at the National Institute of Allergy and Infectious Diseases is already testing the new vaccine in people and says the results of tests in mice, ferrets and monkeys suggest the industry may finally be able to dump the cumbersome process of making fresh flu vaccines every year.

"This is the first step, conceptually, toward a good shot at a universal vaccine," NIAID Director Dr. Anthony Fauci said in a telephone interview.

Every year, the influenza strains that are circulating mutate a little bit and at any given time several very different strains are infecting people. In some years a new mutant pops up -- such as the new H1N1 swine flu strain that appeared in March 2009 to spark a pandemic.

Vaccine makers have to change their formulations every year to match the current strains. To make matters worse, virtually all flu vaccines are made using decades-old technology based on chicken eggs that is both slow and prone to contamination.

So the goal is to come up with a universal influenza vaccine that could protect people from all flu strains for decades or even for life.

NIAID's Dr. Gary Nabel said his lab has taken a big step toward this goal. Their method, described in the journal Science, starts with a piece of DNA based on the hemagglutinin protein -- a mushroom-shaped structure on the outside of the virus that gives flu strains the "H" in their names.

The DNA directs the body to make antibodies against a part of the flu virus that is normally hidden -- on the "stem" of the hemagglutinin protein. This part is conserved, meaning it does not change from flu strain to flu strain.

OLD AND NEW STRAINS

Vaccinated mice and ferrets produced antibodies that protected them against flu strains from 1934 through 2007.

"We are excited by these results," Nabel said. "The prime-boost approach opens a new door to vaccinations for influenza that would be similar to vaccination against such diseases as hepatitis, where we vaccinate early in life and then boost immunity through occasional, additional inoculations in adulthood."

The vaccine, which uses DNA from Netherlands-based Crucell NV, "looks pretty safe," Fauci said. "They are already well into, at least a full year into, a Phase 1 trial."

Such trials are meant to see if a new drug or vaccine is safe in people. A larger, Phase 2 trial could start next year, Fauci said.

Seasonal influenza kills 250,000 to 500,000 people a year globally, including 36,000 in the United States. Pandemics often kill more and while H1N1 has not been especially deadly, it has killed far more children, young adults and pregnant women than seasonal flu usually does.

The experimental vaccine protected animals against H5N1 bird flu, as well. While avian influenza only rarely infects people, it has killed 296 of the 500 sickened by it since 2003.

Flu experts fear H5N1 could mutate and cause a pandemic far worse than swine flu and, using current vaccine technology, it will take months to formulate a good vaccine against it. Swine flu made its spread global within six weeks.

Dozens of companies make influenza vaccines, including Sanofi Aventis, GlaxoSmithKline, Novartis, AstraZeneca and CSL.

(Editing by Eric Walsh)