The government is storing two million doses of bird-flu vaccine and will test how it maintains its potency as U.N. officials warn that if the Asian bird flu mutates it could cause an outbreak that would pose he "gravest possible danger."

Some experts think such an event could kill millions of people.

Officials say the new vaccine was prepared in two different concentrations and is nearly ready to be shipped to the National Institute for Allergy and Infectious Diseases for clinical trials.

Earlier this week the head of the Centers for Disease Control and Prevention, Dr. Julie Gerberding, sparked new concerns when she warned that the virus could mutate to become deadly and infectious, possibly causing a worldwide pandemic she likened to the viruses that killed millions during three influenza pandemics in the 20th century.

Gerberding later clarified her remarks, saying the world is not imminently on the brink of an avian flu pandemic.

But she acknowledges that it's "a worrisome situation."

She says there's concern about the disease as it emerges and she says there's particular concern about Asia.

She says there are more pigs, people and poultry in that environment, and that is the formula for emergence of new flu strains.

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