Such an outbreak has long been called inevitable, once a flu virus circulating in birds infects people and mutates so that it passes easily between people. While bird-to-human infections have been happening in Asia, they have not yet spawned a major spread of the virus person-to-person.

Even an optimistic estimate says a worldwide outbreak could sicken nearly 30 million people enough to need hospitalization, and kill a quarter of those patients, researcher Albert Osterhaus of the Erasmus Medical Center in Rotterdam, The Netherlands, and colleagues write in Thursday's issue of the journal Nature.

They and other experts made recommendations in the journal about how to prepare for a global outbreak, called a pandemic.

"Clearly there is much to be accomplished, and time is of the essence," said Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases.

Osterhaus and co-authors called for a permanent global task force that includes health agencies and researchers from a variety of disciplines.

It should study the global picture of flu in various species, combine knowledge from different research areas, advance strategies for dealing with animal and human outbreaks, and offer policy advice, they wrote.

When an outbreak occurs, task force representatives should join local experts and policy makers on teams to manage the disease, they wrote.

Setting up and operating a global task force would cost less than $1.5 million a year, they estimated.

Michael Osterholm of the University of Minnesota wrote that under current technology, a vaccine for a pandemic flu strain wouldn't be available until at least six months after a pandemic started. And at that point, the supply would cover only 14 percent of the global population, he said.

"We must demand nothing less than an international effort to develop a new type of influenza vaccine that can be manufactured on a much shorter timescale," he wrote.