GENEVA — As swine flu cases topped 6,600 worldwide, vaccine makers and other experts met yesterday at the World Health Organization to discuss the tough decisions that must be made quickly to fight the evolving virus.

Pharmaceutical companies are ready to begin making a swine flu vaccine — but as the virus may mutate, questions abound: How much should be produced? How will it be distributed? Who should get it?

The expert group's recommendations will be passed to WHO Director-General Margaret Chan, who is expected to issue advice to vaccine manufacturers and the World Health Assembly next week.

WHO's flu chief said the meeting of industry representatives and independent experts sought to answer questions including when to recommend to manufacturers that they switch from a seasonal vaccine to one that works against the pandemic strain.

"No big decisions, no announcements," Keiji Fukuda told reporters after the meeting. "These are enormously complicated questions, and they are not something that anyone can make in a single meeting."

But some feel the main decision already has been made.

"It's a foregone conclusion," said David Fedson, a vaccines expert and former professor of medicine at the University of Virginia. "If we don't invest in an H1N1 (swine flu) vaccine, then possibly we could have a reappearance of this virus in a mild, moderate, or catastrophic form and we would have absolutely nothing."

As of yesterday, at least 33 countries reported more than 6,600 cases of swine flu worldwide, with 70 deaths. The figures are based on tallies provided by national governments and WHO. According to the global body's pandemic alert level, the world is at phase 5 — out of a possible 6 — meaning that a global outbreak is "imminent."

North America has been the hardest-hit continent. The United States has reported 3,352 laboratory-confirmed cases of swine flu, including four deaths.