The bird flu vaccine currently being manufactured fought at least two forms of the virus in mice, but it's unclear whether the drug would be equally effective in humans, a health official said Monday.

The vaccine was designed to combat a version of bird flu identified in Vietnam in 2003. Scientists have wondered whether it also offers protection against a version seen later in Indonesia, Europe and Africa.

Experiments have shown that in mice, the vaccine for the first version prevented proliferation of the second, said Dr. Nancy Cox, director of the influenza branch of the federal Centers for Disease Control and Prevention.

But the results can't necessarily be applied to humans, she said. And in an another test that did not involve a vaccine, the immune defenses developed by a ferret against the first version of the bird flu did not work against the second, Cox said.

The test results were discussed Monday at an international conference in Atlanta on emerging infectious diseases.

The H5N1 virus has killed or forced the slaughter of tens of millions of chickens and ducks across Asia since 2003. It has spread more recently with migrating birds to Europe, Africa and the Middle East.

Worldwide, 177 human cases of bird flu have been reported, including 98 deaths, according to researchers at the conference. No cases have been found in the U.S.

Health officials fear the virus could evolve into a version that could easily be transmitted between people, potentially triggering a global pandemic. So far, there has been no evidence of such a mutation.

But more and more versions of H5N1 are being identified, according to research presented Monday.

Scientists must keep a close watch, and it may be necessary to develop additional vaccines as the virus mutates, said Rebecca Garten, a CDC researcher.

The U.S. government is spending $250 million on vaccine doses to fight the Vietnamese version of bird flu and has plans for enough to protect 20 million Americans.

This month, U.S. Health and Human Services Secretary Mike Leavitt authorized work to begin on a second vaccine based on the later form of the virus.