Federal health officials announced plans Monday for a second vaccine to protect people from bird flu because the virus spreading among birds in Asia, Africa and Europe is changing.

The government has several million doses of an earlier bird flu vaccine, but it was based on a sample of virus taken from Vietnam in 2004. The germ is believed to have mutated enough since then that the form now circulating in Africa and Europe may be different, health officials said.

U.S. Health and Human Services Secretary Mike Leavitt said Monday he had authorized the National Institutes of Health and the federal Centers for Disease Control and Prevention to begin working on a second vaccine for humans.

"In order to be prepared, we need to continue to develop new vaccines," Leavitt said at an immunization conference in Atlanta.

Government flu experts wouldn't speculate on whether the earlier vaccine would still protect most people. They said only that they believe it would be less effective than a new vaccine based on a more recent virus sample.

Calls for a second vaccine illustrate the challenge of coming up with an effective shot to protect humans from a strain of bird flu that might one day easily jump to humans. So far, that hasn't happened, but if it did, experts fear a worldwide, deadly flu epidemic.

Because all flu viruses constantly change, even making a vaccine for ordinary human flu poses hurdles.

Health officials plan to base the second vaccine on a sample taken from Indonesia last year, said Ruben Donis, leader of the molecular genetics team at the CDC's influenza branch.

The virus circulating in Indonesia is related to the Vietnamese virus, but it is not a descendant and causes a different immune system response, he said.

A vaccine based on the Vietnamese virus would be protective for people in the Vietnam region, but less effective against viruses circulating elsewhere, Donis said.

He said that perhaps scientists will one day develop a vaccine that protects against several different forms of bird flu.

The U.S. government is already spending $250 million for about 8 million doses against the Vietnamese version of bird flu. Federal officials contracted with two companies — Chiron Corp. and Sanofi Pasteur — for those doses, and most already have been produced, said Bill Hall, a spokesman for Health and Human Services.

The second vaccine must be developed and tested, and HHS had no estimate for the cost of that work.

The World Health Organization has reported at least 174 human cases of bird flu, including 94 deaths since 2003.

So far, most if not all of the human victims were in very close contact with infected birds, but health officials worry that as bird flu spreads, it could mutate into a strain that easily passes among people.

Dr. Margaret Chan, who is spearheading the WHO efforts against the virus, said it poses a greater challenge to the world than any previous infectious disease. Since February, the virus has spread to birds in 17 new countries in Africa, Asia, Europe and the Middle East, Chan said.

Poland on Monday confirmed its first outbreak of the disease, saying laboratory tests found that two wild swans died of the lethal strain.

Several cats have also tested positive for the deadly strain in Austria's first reported case of the disease spreading to an animal other than a bird, officials in that country said Monday.

The WHO describes bird flu as unprecedented in its scope as an animal disease, saying it is costing the world's agriculture industry more than $10 billion and affecting the livelihoods of 300 million farmers.