As swine flu cases topped 6,600 worldwide, vaccine makers and other experts met Thursday at the World Health Organization in Geneva 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 constantly mutates, 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.

But some feel the main decision has already 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."

Most flu vaccine companies can only make one vaccine at a time: seasonal flu vaccine or pandemic vaccine. Production takes months and it is impossible to switch halfway through if health officials make a mistake.

Vaccine makers can make limited amounts of both seasonal flu vaccine and pandemic vaccine — though not at the same time — but they cannot make massive quantities of both because that exceeds manufacturing capacity.

Seasonal flu kills up to 500,000 people a year. At the moment, health officials aren't sure how deadly swine flu is, and whether they will need more seasonal flu vaccine or swine flu vaccine. And if the swine flu mutates, scientists aren't sure how effective a vaccine made now from the current strain will remain.

WHO estimates that up to 2 billion doses of swine flu vaccine could be produced every year, though the first batches wouldn't be available for four to six months.

The U.S. Centers for Disease Control and Prevention is currently working on a "seed stock" to make the vaccine, which should be ready in the next couple of weeks. That will be distributed to manufacturers worldwide so they can start producing the vaccine.

Until vaccine manufacturers get the seed stock, they won't know how many doses of vaccine they can make or how long that would take. Sanofi Pasteur, the world's biggest vaccine producer, said Thursday it is waiting for the green light from WHO before it starts making swine flu vaccine.

WHO is also negotiating with vaccine producers like GlaxoSmithKline PLC to save some of their swine flu vaccine for poorer nations. Many rich nations like Britain, Canada, Denmark, France, Switzerland and the United States signed deals with vaccine makers years ago to guarantee them pandemic vaccines as soon as they're available.

As of Thursday, at least 33 countries reported more than 6,600 cases of swine flu worldwide, with 69 deaths. According to WHO's pandemic alert level, the world is at phase 5 — out of a possible 6 — meaning that a global outbreak is "imminent."

"It's a no-brainer," Fedson said of the decision to make swine flu vaccine. "All that's being discussed now is the details of how to make sure you have enough seasonal flu vaccine and the logistics of making the switch to H1N1 vaccine production."

North America has been the hardest-hit continent. The United States has reported 3,352 laboratory-confirmed cases of swine flu, including three deaths. Mexico has 2,656 cases and 64 deaths, while Canada has 389 cases with one death, according to WHO figures.

Mexico confirmed 374 more cases Thursday including four more deaths, but Health Secretary Jose Angel Cordova said the new cases show the virus is appearing less deadly. Mexico's swine flu deaths now represent 2.4 percent of its confirmed cases, he said.

Spain and Britain have the most cases in Europe, at 100 and 78 respectively.

In Central America, Costa Rica has eight cases and one death and Panama has 29 cases.