Countries around the world need to start rehearsing plans for tackling a human flu pandemic to identify hidden obstacles and ensure the best response once the virus arrives, the World Health Organization said Monday.

Health experts at the first major international coordination meeting on bird flu and human flu urged countries that haven't done so to draw up plans for handling an inevitable new pandemic, which the World Bank estimated could result in more than $800 billion in lost gross domestic product over a single year.

Experts agree a global flu outbreak capable of killing millions of people is a certainty.

What is also certain, say scientists, is that the virus will come from bird flu. But what is unknown is whether the H5N1 strain that has ravaged poultry stocks in parts of Asia and spread through Eastern Europe will be the culprit.

It is the leading candidate, however, and authorities are trying to stamp out poultry outbreaks as rapidly as possible to reduce opportunities for the virus to mutate into a form that can pass easily between people and spread worldwide. Currently, the virus is hard for people to catch, and most deaths have been linked to human handling of infected poultry.

The World Health Organization has been urging countries to draw up pandemic flu plans for almost a decade, but many did not act until the bird flu outbreak in Asia became an apparent threat.

The H5N1 virus has killed at least 63 people across Southeast Asia since 2003, mostly in Vietnam. On Tuesday, Vietnam confirmed its 42nd human death — a 35-year-old man who became sick after his family bought a chicken from a market in Hanoi.

Six months ago, fewer than 40 countries had a strategy, said Dr. Mike Ryan, director of epidemic and pandemic alert and response at WHO. Now, 120 countries, or 60 percent of the WHO member states, have planned responses.

"That's pretty unprecedented in public health, but we need to push it further, into the implementation, the rehearsing and testing," Ryan said.

"A national plan is like a New Year's resolution. You can write it down on paper, but the question is do you actually carry it out? ... You've got to get money, get the training going. In the end you've got to test it and see if it actually works."

The plans contain commitments to activities such as improving early detection of disease, increasing the ability of hospitals to cope with an influx of patients and the intention to stockpile drugs, pre-order vaccines and quarantine communities.

The more real the rehearsal, the better, Ryan said.

One testing option is to have the various players sit around a table and work through various scenarios. The European Union is planning such an exercise next month.

Another option is a war game, where the scenario is acted out. However, that costs more money and can disrupt health systems.

"It's only when you run a simulation that you realize you don't have the guidelines for this, or you won't be able to deploy people from here to there, or there were legal obstacles to quarantine. You start to see the weaknesses in your system," Ryan said. "At the very least, national plans should be tabletop exercised."

Still, he said the level of preparedness already and the meeting to forge a global strategy are unprecedented.

"It's the first time the international community have come together before we've had the disaster. It's always after the tsunami, after SARS, after AIDS," Ryan said.

Several developing countries — Morocco, Kenya, Indonesia, India and Thailand — said Monday they were worried about getting drugs and human vaccines because they lack pharmaceutical manufacturing or money to buy products.

Pharmaceutical groups were also meeting in Geneva to develop a coordinated vaccine strategy for the industry.

"If something happens tomorrow, we are very poorly prepared," said Dr. Bram Palache, medical director of Belgian pharmaceuticals and chemicals giant Solvay Pharmaceuticals. "There is no doubt about that. We don't have the means to really properly do the interventions which will be needed."

Money will be needed to prepare the world, experts said.

"The potential cost of this thing is in the multibillions," said Dr. David Nabarro, the U.N. coordinator for bird and human flu. "We are asking for what would be a quite small fraction of that. This is not a begging bowl. This is an investment in the future of the world, an investment in the future of society."