Swine Flu Likely to Return to U.S. Next Winter

"We've been very lucky so far that it's appearing to be mild, at least in the U.S., as far as virulence and susceptibility to antivirals is concerned," he said. "It could change. That's one of the concerns."

"The severity depends on whether [and how] the genotype of the virus reassorts itself," Horovitz added. The reassortment may be so minimal as to make no clinical difference, or it could assert itself in entirely new ways."

But a vaccine will likely be ready by the time a second wave hits, Ostrosky noted, and the world is prepared in other ways as well.

"We have completely sequenced the genome of the virus, and it shows low virulence at this point. We know about it. We can prepare," he said. "If nothing else, this has been an extraordinary exercise in preparation."

More information

The U.S. Centers for Disease Control and Prevention has more on the current outbreak of swine flu.