Here's how the seasonal formulation occurs. First, more than 120 sites around the world work year-round collecting data on which strains are circulating. Twice annually, a panel of experts meets to determine which strains should be included in that season's vaccine.
So, first researchers need to know more about what strains of swine flu are circulating, Quarles said. And other questions remain unanswered -- Why are cases apparently more lethal in Mexico? How are the strains evolving?
"There's a lot of basic work they need to do over the next two-to-three weeks," Quarles said. "It's just too early to have a hold on this."
But much of that work may be under way. On Tuesday, Dr. Ruben Donis, swine flu chief at the U.S. Centers for Disease Control and Prevention, said that engineers there must first devise a strain of swine flu that will spur the immune system without causing real illness. Donis told the Associated Press that his team is about a third of the way to creating such a strain.
And at the U.S. Food and Drug Administration, Dr. Jesse Goodman, who's heading that agency's swine flu effort, told the AP that his group is working "at 100 miles an hour" to create quality raw materials to deliver to vaccine manufacturers.
But even with the "regular" flu, there is often not a perfect match between circulating strains and the distributed vaccine. However, people receiving a non-perfectly matched vaccine can still use it to help avoid getting sick, or at least not be sick for as long, Quarles said.
And there's another key challenge to vaccine production, the experts said. The U.S. still uses a relatively archaic egg-based production technique, in which selected strains of the influenza virus are grown in chicken eggs. Often, one egg produces just two doses of a vaccine.
In the case of an experimental avian flu vaccine, Stanberry said, one eg
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