Data presented Tuesday at an international conference on bird flu indicate that drug manufacturers could produce enough influenza vaccine starting sometime in 2008 to protect between a quarter and half the world's population over the course of a year, should a new and dangerous strain of flu virus emerge.

The optimistic projection is based on information provided at a meeting called by the World Health Organization and three other international agencies to prepare for a possible influenza pandemic. The projection assumes that the bird flu virus now circulating in Asia and Europe -- or some other exotic strain -- will not be able to infect people easily for at least several years.

The virulent strain of the virus, known as H5N1, has infected 124 people and killed 63, according to WHO statistics. But so far, almost all of the people who have contracted the disease had close contact with infected birds. The virus does not pass easily among humans, but specialists say it could gain that capacity over time through mutation or recombination with other strains of flu, eventually leading to a pandemic.

For a vaccine to reach a quarter to a half of the world's population -- 1.7 billion to 3.4 billion people -- current plans by manufacturers to expand production would have to go forward uninterrupted. In order to immunize the maximum number of people, doses would also have to be used with immunity-boosting substances that would make the vaccine more effective. The projections assume there will be enough trained technicians and needles on hand to administer the vaccines.

The projection of reaching up to 3.4 billion people also assumes the use of a technique that would double the vaccine's potency, based on research being conducted.

Few specialists think that everything will go so smoothly. WHO, which has analyzed global vaccine production, is unwilling to certify the projection of 3.4 billion vaccinations. Nevertheless, even the projection of 1.7 billion represents a major expansion over the current annual production of 300 million seasonal flu shots.