Fear of the bird flu sweeping across Asia has played a major role in driving the government's flurry of preparations for a worldwide epidemic.

That concern prompted President Bush to meet Friday with vaccinemakers to try to persuade them to step up production, and it led the Health and Human Services secretary, Michael O. Leavitt, to depart Saturday for a 10-day trip to at least four Asian nations to discuss planning for a pandemic flu.

But scientists say that although the threat from the current avian virus is real, it is probably not immediate.

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said a bird flu pandemic was unlikely this year.

"How unlikely, I can't quantitate it," Fauci said. But, he added, "You must prepare for the worst-case scenario. To do anything less would be irresponsible."

"I would not say it's imminent or inevitable," said Dr. Jeffery Taubenberger, chief of the molecular pathology department at the Armed Forces Institute of Pathology. "I think in the future there will be a pandemic." But, he added, whether that pandemic will be bird flu or another type, no one can say.

The Bush administration is in the final stages of preparing a plan to deal with pandemic flu. A draft shows that the country is woefully unprepared, and it warns that a severe pandemic will kill millions, overwhelm hospitals and disrupt much of the nation.

What worries scientists about the current strain of bird flu, known as H5N1, is that it has shown some ominous traits. Though it does not often infect humans, it can, and when it does, it seems to be uncommonly lethal. It has killed 60 people of the 116 known to have been infected.

Alarm heightened on Thursday when a scientific team led by Taubenberger reported that the 1918 flu virus, which killed 50 million people worldwide, was also a bird flu that jumped directly to humans.

But there is a crucial difference: The 1918 flu was highly contagious, while today's bird flu has so far shown little ability to spread from person to person. But a mutation making the virus more transmissible could set the stage for a pandemic.

Another concern is that H5N1 has become widespread, killing millions of birds in 11 countries and widely dispersing as migratory birds carry it even greater distances. This month, it was reported in Romania.

Meanwhile, it is spreading widely among birds in Asia. And it has unusual staying power: it has persisted in different parts of the world since it emerged in 1997.