Global demand for a yet-to-be-developed swine flu vaccine could overwhelm manufacturers, experts say, and because much of the U.S. flu vaccine supply is made overseas, this could make it tough for some Americans to get immunized. 

Right now, there are only about 260 confirmed cases worldwide, with 109 in the United States. 

But the World Health Organization on Wednesday raised the pandemic alert level to one notch below its highest level. If the outbreak spreads and grows more severe, health agencies want to be prepared to inoculate as many people as possible. 

The U.S. Centers for Disease Control is looking "very intently" at pursuing a vaccine. The World Health Organization already has reached out to flu vaccine developers about the possibility of helping produce one for this unique strain, known as H1N1 influenza A. 

Experts warn, however, that even if a new vaccine can be produced over the next several months there may not be enough of it to go around.

"The assumption is there is nowhere near the capacity that's needed to vaccinate the world's population," said Neal Halsey, international health professor at Johns Hopkins Bloomberg School of Public Health. He said all the manufacturers in the world could probably only produce enough vaccine for 10 to 20 percent of the global population. 

He said if the strain leads to a higher-than-usual mortality rate, the demand in the United States alone would probably be at least 200 million doses. 

And he said the United States wouldn't necessarily have the upper hand in seeking a hoard of those doses, since much of our vaccine manufacturing is done overseas -- in countries like France, Belgium, Germany and Great Britain. 

"That's going to be a very delicate discussion and argument," Halsey said, when asked about how the vaccines could be distributed globally. "The countries where the manufacturing takes place are going to have some say in this." 

Six manufacturers are responsible for churning out the United States' annual seasonal flu vaccines. One of them, Sanofi Pasteur, released a statement Wednesday saying it stands ready to develop a new vaccine for the latest outbreak. The company has locations in France, Canada and the United States. 

Kathryn Edwards, pediatrics professor at Vanderbilt University Medical Center, said Sanofi Pasteur still produces a lot of its vaccine supply in the United States. But she said those infected with the new strain of influenza could very well need two doses, not one, of any new vaccine -- in turn making it harder to satisfy the demand in the United States and globally. 

"I think that that will be a challenge," she said. "I think it's fair to say that during a pandemic that people are going to be pretty parochial. They're going to make a vaccine for their people first." 

Anne Schuchat, an interim deputy director at the CDC, tried to dispel concerns about U.S. manufacturing capacity during a Senate committee hearing Wednesday.

Asked if the United States can produce a vaccine domestically for 300 million Americans, she said recent investments in pandemic preparedness had led to "phenomenal expansion in manufacturing capacity so that we are very optimistic going forward about what we can expect." 

But she added: "This virus can surprise us, and even with all this investment, it may just technically be difficult." 

At least one U.S. company has jumped in early to start evaluating the strain, potentially in order to develop a vaccine. Illinois-based Baxter International Inc. requested virus samples from the World Health Organization and expects to obtain them in the next few days, company spokesman Chris Bona said. 

He said Baxter has a special system to "rapidly produce" flu vaccines and potentially could develop one in half the time it normally takes -- about 26 weeks. 

If Baxter develops a vaccine, Bona said, it would work with "global health authorities" to determine where it would be distributed. He would not provide details about how many doses Baxter might be able to develop. 

Halsey doubted Baxter could produce a substantive supply of vaccines for the H1N1 strain. 

Richard Besser, acting director of the CDC, said Tuesday that the agency was looking "very intently" at a new vaccine. He said the CDC had built up a "seed stock" of the virus so it is ready to manufacture a vaccine if necessary. 

"We're moving forward aggressively so that if a decision is made that we need to rev up production to make that vaccine, we would be ready to do so," he said. "It will be a matter of us deciding not to make a vaccine rather than deciding to move forward. What we're doing now is very proactive in terms of growing up the seed stocks for a vaccine."