The U.S. is unprepared for the next flu pandemic, lacking the manufacturing capacity to provide 300 million doses of a vaccine for three to five more years, Health and Human Services Secretary Mike Leavitt said Sunday.

"What we all learned from (Hurricane) Katrina is that sometimes we have to think very clearly about the unthinkable," Leavitt said. "We're not as prepared as we need to be. ...We will not have enough for everyone."

A strain of a bird flu that has killed 67 people in Asia has sparked concerns of a super-flu that could kill millions worldwide, and U.S. officials acknowledge that the strain in its current form could reach here through a migratory bird.

While stressing that chances remain slight, health experts say it could lead to a global pandemic if the bird flu mutates to start spreading easily among people.

"We can't put a number on how probable that's going to be," said Dr. Anthony Fauci, director of the infectious disease division of the National Institutes of Health. "It's a low probability. When the consequences are unimaginable, you must assume the worst-case scenario."

Added Dr. Michael Ryan of the World Health Organization: "This is certainly a dangerous virus, and it has crossed the species barrier now in 130 cases. We're probably closer to a pandemic at any time in the last 37 years."

The U.S., which has not seen any signs of the strain in birds or people, has only enough doses now for 4.3 million people.

President Bush has proposed stockpiling enough of the anti-flu drugs Tamiflu and Relenza for 81 million people, a goal drug manufacturers believe they can reach by the middle of next year, said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.

"We're not prepared for vaccination, that's why we need to scale up. We are doing studies to extend the value of the vaccine ... allowing us to vaccinate more people with the same doses," so the timeframe might be quicker, she said.

Leavitt said the low supply means state and local governments will have to make tough choices on how best to allocate the vaccine should an outbreak occur. The federal government has suggested top priority be given to first responders.

The first documented cases of bird flu in people occurred in 1997 in Hong Kong, where six people died. The current flu virus strain appeared in people in 2003. More than 150 million chickens and ducks have died from the germ or been slaughtered.

Nearly all of the 67 human victims caught the virus from close contact with sick chickens, with only one confirmed case of a person infecting another person. The fear now is that the strain, called H5N1, will acquire the ability to spread easily from person to person.

Fauci said the chances of that remain slight, noting that the strain will have to genetically mutate in ways that are possible but "not necessarily inevitable."

"We know it can jump from a chicken to a human," he said. "If this virus was the seasonal flu with the inherent capability that the seasonal flu has of going from human to human, you would have seen an explosion of cases in Southeast Asia. ...We're not seeing that now."

Ryan said his group is working to improve health surveillance in Asia, which he called the weakest link, particularly since health experts are preparing to provide an emergency "fire blanket" to control an outbreak should one occur.

"If we were to detect the emergence of the pandemic strain early enough, some models suggest that with the application of social distancing or quarantine-like measures and the rapid distribution of antivirals in that population, we may be able to significantly slow down or even stop the emergence of a pandemic strain," he said.

Gerberding advised that Americans should take the usual precautions in guarding against the common flu, such as washing hands frequently and getting a flu shot.

"HFN1 is a bird problem, and it's not in the United States at this time," she said. "Even if it does enter through a migratory bird at some point, which won't be surprising, we have a wonderful system of surveillance."

The four health experts appeared on NBC's "Meet the Press."