The World Health Organization issued an optimistic update on prospects for pandemic influenza vaccine Tuesday. But industry insiders and public health experts wondered if tinted glasses were casting an overly rosy glow over the global vaccine production situation.

And some openly admitted the efforts to grow production potential could soon lead to a glut in seasonal flu vaccine stockpiles.

"We are very concerned about an overcapacity situation,'' said Len Lavenda, a spokesperson for the industry's biggest player, sanofi pasteur.

At a news conference in Geneva, the senior WHO official in charge of the pandemic vaccine file said the effect of expanded manufacturing capacity for seasonal flu vaccine, coupled with the success of vaccine-stretching compounds known as adjuvants, means several billion more people could be vaccinated if a pandemic occurs in the next few years.

Where a year ago, only 100 million people could have been vaccinated in the first year of a pandemic, by 2010 that number could rise to 4.5 billion people, said Dr. Marie-Paule Kieny, director of the vaccine research initiative.

"With influenza vaccine production capacity on the rise, we are beginning to be in a much better position vis-a-vis the threat of an influenza pandemic,'' Kieny said in a statement, adding that the WHO believes the goal should be enough vaccine to protect 6.7 billion people in the first six months of a pandemic.

But observers noted that in order to reach even the 4.5 billion figure, global output of seasonal flu vaccine will have expand to one billion doses from the current capacity of about 565 million.

And given that the world currently discards millions of unused flu vaccine doses every year, many wonder how production could reach or be sustained at the one billion dose level, let alone go higher.

It's a cause for concern for the industry.

"Absolutely. Absolutely,'' Lavenda said from sanofi's U.S. headquarters in Swiftwater, Pa.

"I can only speak for sanofi pasteur. But we're the largest supplier. We're producing over 40 per cent of the world's flu vaccine. And we are very concerned about an overcapacity situation.''

"That's not healthy for industry.''

He said overproduction could drive some producers out of the field. He pointed to the recent past, when Wyeth Pharmaceuticals withdrew from the flu vaccine production market in 2002 rather than incur the cost of upgrading an aging plant.

"You don't have to turn the history book back too many chapters to see that that's real, that's not idle, that's not just a theoretical possibility. That's reality,'' Lavenda said.

An industry association director also questioned the sustainability of a one billion dose a year flu vaccine market.

"If there is enough demand they are ready to increase it up to about one billion _ that's max, max, max. You're talking about 24 hours, seven days a week, 12 month whole production,'' said Dr. Ryoko Krause, of the International Federation of Pharmaceutical Manufacturers and Associations.

"But if there's no demand it's not going to increase,'' she said from Geneva. "We have always said that there's no way that the industry is going to keep producing or keep the capacity unused just for the potential of creating one billion doses.''

Lavenda said a key to expanding the production capacity would be a move by the United States to recommend everyone in that country get an annual flu shot.

Dr. John Treanor, an influenza vaccine expert at the University of Rochester, N.Y., said he believes the U.S. is moving in that direction. But even if the country's public health advisers take that move, experience elsewhere suggests that might translate into about half of Americans agreeing to take a flu shot each year.

In Ontario, the first jurisdiction in the world to provide free flu shots to anyone who wants them, only about 42 per cent of people actually take up the offer.

The United States, one of the biggest users internationally of flu vaccine, throws out millions of unused doses at the end of every flu season. In 2004 -- the year when nearly half the country's flu shot supply had to be discarded because of contamination and thousands of Americans flooded over the border looking for shots in Canada -- 4.5 million doses were eventually thrown out.

If developed countries, which can afford flu vaccine, can't vaccinate a greater percentage of their populations, can the world really expect developing countries to squeeze out scarce funds to buy seasonal flu vaccine for their people? Especially when they have so many other more pressing health needs?

Treanor insisted a persuasive case could be made in some developing countries.

"I do believe that there is an opportunity to expand the global market for seasonal flu (vaccine) far beyond what it is now. Now whether it could really get up to a billion doses . . . I don't know. But it could be higher than it is now.''

Some worry that building the pandemic flu production capacity on the backbone of the seasonal flu vaccine output leaves the globe vulnerable to the whims of market forces.

"To ask industry to be industry some times but industry not to be industry other times is illogical and inconsistent and won't happen,'' said infectious diseases expert Dr. Michael Osterholm.

"We all want to dramatically increase the production capability for the influenza vaccine of the future. But this approach sounds neither realistic nor practical.''

"When we can get 95 per cent of health-care workers vaccinated in the United States, then talk to me about the rest of the world,'' added Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.

He noted less than 50 per cent of U.S. health-care workers get vaccinated against the flu "and that's after years of trying.''