SAN FRANCISCO (AP) -- A leading flu expert says an encouraging but small study in mice gives more reason to hope that ordinary flu shots might help protect people if bird flu starts spreading among humans and causes a pandemic.

Half of the mice given an ingredient from the regular flu shot survived infection with bird flu in the first experiment of its kind, Dr. Robert Webster of the St. Jude Children's Research Hospital in Memphis, Tenn., said Friday at an infectious diseases conference.

Experts said the work supports the wisdom of getting regular flu shots, but cautioned against too much optimism that they would protect against bird flu.

"We need a lot more data from humans" before saying that might work, said Dr. Frederick Hayden, a University of Virginia virologist on special assignment with the World Health Organization.

The experiment on about a dozen mice was led by Webster's colleague, Richard Webby.

Regular flu vaccine contains the same 'N' ingredient as the deadly H5N1 bird flu strain, and many people also have been naturally exposed to it from seasonal flu outbreaks over the last several decades. Some scientists think that might help people survive bird flu, even though it wouldn't prevent infection because the 'H' parts are so different.

The mice results "suggest there is some basic cross-protection," Webster said, and that it makes sense for countries to stockpile ordinary vaccine while those against bird flu are being developed.

However, the type of vaccine ingredient the mice were given does not work as well in people, so it's unclear how well the results would apply, said Dr. Wendy Keitel, a vaccine expert at Baylor College of Medicine in Houston.

Dr. Kathy Edwards, a vaccine scientist at Vanderbilt University in Nashville, said routine flu shots still make sense because they cut the risk people will become infected with multiple flu strains.

She said that would help people avoid what can happen in pigs, which can get two viruses "that have the ability to reassort" and form a killer strain.

Webster also told scientists at the conference that ducks increasingly look like a potential "Trojan horse of influenza."

Increasingly, ducks are not dying once infected with bird flu. That leaves them better able to spread it. Research also suggests they now can shed the virus through respiratory secretions, not just droppings.

That means that in live poultry markets "humans can breathe these viruses and not just pick them up on their shoes," he said.

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