As of Tuesday morning, Fukuda said, there were 26,563 laboratory-confirmed cases of H1N1 flu, or swine flu, in 73 countries, with 140 deaths. In the United States, the most recent figures from the national Centers for Disease Control and Prevention showed 13,217 confirmed cases and 27 deaths.
Officials believe the number of cases, both in this country and worldwide, is actually much higher because many people have mild cases and are not tested.
Fukuda said that researchers have seen few, if any, changes in the virus since its emergence in Mexico early this year, and that it still remains susceptible to the antiviral drug Tamiflu.
But they are concerned that infections continue in North America and Europe, even though the traditional flu season has ended in the Northern Hemisphere.
"The disease patterns are not what we see from seasonal influenza," he said. That suggests the virus has greater capability for spread than does the seasonal flu.
Most of the infections have been in people younger than 60, which is also different from seasonal flu. That suggests, some experts said, that older people may have been exposed at some point to a different swine flu virus that has conferred some immunity.
About half of the people who have died of the virus were previously healthy, with no underlying medical conditions.
"That is one of the observations that has given us the most concern," Fukuda said. "We don't know why they died and why other people recovered."
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thomas.maugh@latimes.com
