After four weeks, almost 1,700 people in the U.S. would have symptoms, including 198 in Los Angeles, according to his model. That's just a fraction of the county's thousands of yearly flu victims.
Just because the virus is being identified in a growing number of places -- including Austria, Canada, Germany, Israel, New Zealand, Spain and Britain -- doesn't mean it's spreading particularly quickly, Olsen said.
For The Record
Los Angeles Times Friday, May 01, 2009 Home Edition Main News Part A Page 4 National Desk 1 inches; 56 words Type of Material: Correction
Swine flu: An article in Thursday's Section A about the risks posed by swine flu said that in the United States annually "between 5% and 20% of the population becomes ill from the flu and 36,000 people die -- a mortality rate of between 0.24% and 0.96%." The correct mortality rate is between 0.06% and 0.24%.
"You don't ever find anything that you don't look for," he said. "Now that diagnostic laboratories and physicians and other healthcare workers know to look for it, perhaps it's not surprising that you're going to see additional cases identified."
And a pandemic doesn't necessarily have a high fatality rate. Even in Mexico, the fatalities may simply reflect that hundreds of thousands of people have been infected. Since the symptoms of swine flu are identical to those of a normal flu, there's no way to know how many cases have evaded government health officials, St. Jude's Webby said.
As the virus adapts to its human hosts, it is likely to find ways of spreading more efficiently. But evolution also suggests it might become less dangerous, Olsen said.
"If it kills off all its potential hosts, you reach a point where the virus can't survive," he said. Working to calm public fears, U.S. officials on Wednesday repeatedly stressed the statistic of yearly flu deaths -- 36,000.
Sebelius and Homeland Security Secretary Janet Napolitano also rejected calls to close the borders, which several lawmakers reiterated Wednesday on Capitol Hill.
"We are making all of our decisions based on the science and the epidemiology," Napolitano said. "The CDC, the public health community and the World Health Organization all have said that closing out nation's borders is not merited here."
Though scientists have begun to relax about the initial toll, they're considerably less comfortable when taking into account the fall flu season. They remain haunted by the experience of 1918, when the relatively mild first wave of flu was followed several months later by a more aggressive wave.
The longer the virus survives, the more chances it has to mutate into a deadlier form.
"If this virus keep going through our summer," Palese said, "I would be very concerned."
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karen.kaplan@latimes.com
alan.zarembo@latimes.com
Staff writers Noam Levey in Washington, Thomas H. Maugh II in Los Angeles and Ken Ellingwood in Mexico City contributed to this report.
