Swine flu virus spreads

NEW YORK — With 20 human swine flu cases confirmed in five states, the Obama administration on Sunday declared a “public health emergency in the United States.”

The emergency allows federal health authorities to tap the national stockpile of anti-viral medications and make other preparations in case the outbreak of human swine flu cases develops into a more widespread epidemic.

In Geneva, Margaret Chan, director-general of the World Health Organization formally declared a “Public Health Emergency of International Concern,” but the WHO has not so far raised the pandemic threat alert level from its current phase 3 (new virus causes human cases: no or limited human-to-human transmission) to phase 4 (new virus causes human cases: evidence of increased human-to-human transmission).

Although no fatalities have been reported in the U.S., the latest reports from Mexico suggest that more than 100 people have died and at least 1,400 may have been infected with the never-before-seen flu.

On Sunday, Canadian health officials confirmed six cases of human swine flu — four in Nova Scotia  and two in British Columbia — while public health officials in New Zealand, Israel, France and Spain began testing several patients with flu-like illnesses who had recently traveled to Mexico to determine whether or not they also had swine flu.

Part of what concerns health officials is that most of the fatalities in Mexico have involved adults under the age of 60, which does not fit the usual profile of a seasonal flu outbreak. That pattern is, however, reminiscent of the flu pandemic of 1918-1919, in which the very young and the very old tended to be spared while most of the fatalities occurred in adults in their 20s to 50s.

Still unclear, however, is whether the Mexico pattern will hold up in other countries. Or even whether it is the best explanation of what is happening in Mexico. It may turn out, for example, that the deaths only appear to be clustered among middle-aged adults because they were the ones who went to the hospital or because deaths among older people have not yet been studied closely enough because they are more common.

Another reason for ramping up possible pandemic preparations is that laboratory tests have now confirmed that this new strain of flu has not been seen before in either humans or pigs.

That last piece of information requires a little unpacking. Flu viruses are identified on the basis of the genes they contain. Technically speaking, the current human swine flu is a new strain of a very widespread sub-type, dubbed H1N1. Indeed, the current 2008-2009 seasonal flu vaccine already protects against a different strain of H1N1 that was identified in Brisbane in 2007.

But even though the current seasonal flu vaccine already protects against the Brisbane H1N1 strain of flu, health officials believe it probably won’t protect against the new H1N1 human swine flu because the two strains are too different from each other. The new human swine flu has genetic components from birds and swine that simply aren’t found in any other H1N1 strains that we know about.

Unfortunately, knowing a flu virus’s genetic makeup is not enough to make accurate predictions about whether it will cause widespread disease.

“Flu viruses are extremely unpredictable and variable,” Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, told the press briefing at the White House on Sunday. “Outbreaks of infectious disease are extremely unpredictable and variable. And so over time what we say about this and what we learn will change.”

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