For every confirmed case, however, there are probably hundreds of unconfirmed cases, noted Dr. Gordon Dickinson, chief of infectious diseases at the University of Miami Miller School of Medicine and the Miami VA.
Three influenza pandemics swept across the globe in the 20th century: the so-called Spanish flu epidemic of 1918, the "Asian" flu in 1957 and "Hong Kong" flu in 1968. Pandemics are labeled as such whenever a new flu virus appears and has sustained human-to-human transmission.
Based on an analysis of confirmed swine flu cases in Mexico and its international spread, the WHO researchers estimate that 23,000 individuals (possibly up to 32,000) had been infected in Mexico alone by late April, with a death rate of about 0.4 percent.
That's much less lethal than the 1918 pandemic, which killed 2.5 percent of people infected.
But it may be too early to breathe a sigh of relief, experts said. The swine flu outbreak could be dying down or it could simply gearing up for a deadlier resurgence in the fall and winter.
"It's too early to say," Dumyati said. "At this point, it doesn't look like it has sustained transmission, but you don't know, in the winter, what will happen."
"People need to stay tuned and see what's going on, and the medical community needs to work fast and hopefully, a vaccine will be prepared," Dickinson added. "What bothers me is that, in 1918, the influenza emerged in the spring then sort of melted away during the summer. Then it came back as a very lethal process. This gives one pause as we see this current H1N1 epidemic spreading . . . The cases continue to emerge. It just hasn't hit with a major force yet."
U.S. public health departments are so overwhelmed, Dickinson said, that they are asking facilities not to send samples for testing unless there is a cluster of outbreaks or a particular
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