ATLANTA -- With new versions of bird flu emerging, U.S. health officials announced Monday that scientists must stir up a different vaccine recipe to try to protect people.
That's not unexpected because flu viruses -- whether in birds or people -- are constantly changing.
Federal health officials are merely trying "to keep right on the virus's tail and keep our vaccines as up to date as much as we can," said Dr. William Schaffner, a Vanderbilt University vaccine expert.
But despite its mutations, the continent-hopping bird flu virus seems content slaying wild birds and farm chickens, causing an estimated $10 billion in global agricultural losses.
It still doesn't easily infect people. That's good news, right?
Not necessarily, said Schaffner, who suggested three possible scenarios.
The virus could continue to spread in its current forms, mostly sparing humans. It could mutate into a more harmless version, which isn't even dangerous to birds. Or it could become a deadly human flu that spreads easily around the globe with the potential to kill millions, he said.
"We cannot let our guard down, because a series of genetic changes could happen at any time that could allow this virus to pick up the capacity to move from person to person," Schaffner said.
That's the thinking that led to Monday's announcement by U.S. Health and Human Services Secretary Mike Leavitt. He authorized the National Institutes of Health and the federal Centers for Disease Control and Prevention to begin working on a second vaccine to protect people.
The government already has several million doses of a bird flu vaccine based on a sample of a virus taken in 2004 from Vietnam, where the first spate of cases in people began to show up.
But researchers have noted the emergence of a second version of bird flu. Under Leavitt's direction, U.S. researchers plan to create a new vaccine targeted at the second variety.
"In order to be prepared, we need to continue to develop new vaccines," Leavitt said at an immunization conference in Atlanta.
The second vaccine will be based on a virus 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 it causes a different immune system response, he said.
The U.S. government is already spending $250 million for about 8 million doses against the Vietnamese version of bird flu, but has plans for enough vaccine to protect 20 million Americans.
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. Researchers are signing up volunteers to test the vaccine.
The second vaccine must be developed and tested, and health officials had no immediate estimate for the cost of that work or the time frame for producing a new vaccine.
The vaccine based on the Vietnamese virus would be protective for people in the Vietnam region, but less effective against viruses circulating elsewhere, health officials said.
For most parts of the world, Schaffner believes it would be "partially effective."
How effective? Scientists don't have enough data yet to make a good prediction, he said.
No matter how effective, work on the first vaccine -- and now this second version -- should benefit subsequent efforts, said Dr. Walt Orenstein, associate director of Emory University's Vaccine Center.
"It's not like this will get made and sit on the shelf and forget about it. Many things may be learned," Orenstein said.
Since 2003, the World Health Organization has reported at least 174 human cases of bird flu, including 94 deaths. Most, if not all, of the victims were in very close contact with infected birds, but health officials worry that as bird flu spreads, its chances of mutating into a form threatening to people increase.
Health officials probably don't know how many people have been infected, and it's possible that bird flu does not kill as many of its human victims as some data have suggested, said Dr. Andrew Pavia, an infectious disease specialist at the University of Utah.
Some scientists think that if it becomes more efficient at spreading among humans, it will lose some of its efficiency at killing. "But that's still speculative," Pavia said.
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.
In Austria, several cats tested positive for the deadly strain, officials in that country said Monday. Last week German officials reported a cat -- infected from apparently eating a sick bird -- and warned pet owners in some areas to keep their cats indoors and their dogs on a leash.
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.