Experts Study Differences in Flu’s Severity

Health authorities raced yesterday to unravel the many mysteries about the ominous new swine flu spreading around the world, including how widely the virus might cause the severe form of illness that so far has been restricted to the epicenter of the outbreak in Mexico.

As the number of confirmed infections in the United States jumped again and cases were confirmed for the first time in Britain, New Zealand and Israel, researchers searched for clues as to how readily the virus causes the pneumonia that has hospitalized and killed patients in Mexico. Only a handful of patients in the United States and elsewhere outside Mexico have been hospitalized, severe complications have been relatively rare, and no one has died.

“We still do not have a good explanation for why the pattern of cases in other countries appear relatively mild while the pattern of cases in Mexico appear to be much more severe,” said Keiji Fukuda of the World Health Organization. “This will be the object of a great deal of research and attention, but at this time, we can’t say why there appears to be a difference.”

Experts said there are several possibilities: Victims in Mexico may be more vulnerable because of nutritional deficiencies, other infections or some other factor; medical care may be better in the United States and elsewhere; the virus could be weakening as it spreads; or too few cases may have occurred outside Mexico for severe illnesses to emerge.

“This is the mystery,” said Arnold Monto, an influenza expert at the University of Michigan. “You could speculate about so many things. It’s an incredibly important question.”

Several officials said some deaths outside Mexico are probably inevitable.

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