As the World Health Organization declared a global flu pandemic Thursday, raising the alert to its highest level, federal health officials said it was unclear whether an effective vaccine would be available by fall.
Federal and local health officials are eyeing the Southern Hemisphere, where the virus is already on an unstoppable course and where it's feared it might combine with the seasonal flu strain and develop drug resistance.
The U.S. government has invested $1 billion toward vaccine production. But a vaccine must first be tested in a federally overseen clinical trial. No one knows whether the newly commissioned vaccine _ in its earliest phases of development _ will pass critical scientific testing. The first bird flu vaccine developed three years ago did not initially meet expectations.
"Vaccines are not the only tools we have in the toolbox," Dr. Anne Schuchat of the Centers for Disease Control and Prevention said Thursday. "We have to be ready for the idea that we may not get a vaccine as soon as we'd like it, or we may not get a vaccine that works as well as we would like it. Or we might not get a vaccine."
Once a vaccine passes clinical tests, government officials will decide to whom it should be made available and whether it should be included in the Strategic National Stockpile, the cache of medications and supplies used in national emergencies.
WHO's declaration follows communitywide spread of the H1N1 strain, particularly in Australia, where the regular flu season is just getting under way. Global health leaders said the geographic spread meant all criteria had been met to declare a flu pandemic, the first in 41 years.
"The declaration of a pandemic does not suggest there's been any change in the behavior of the virus, only that it is spreading in more parts of the world," said Dr. Thomas Frieden, CDC director.
"Really, for all intents and purposes," Frieden said, "the U.S. government has been in Phase 6 of the pandemic for some time now."
H1N1 has now spread to 74 countries, infected at least 28,774 people and caused 144 deaths _ 10.4 percent of which have occurred in New York City.
The greatest concentration of cases has been in the New York/New Jersey region and in New England, with children and young adults composing the largest number of serious cases and hospitalizations.
Flu-tracking scientists from the CDC have fanned out below the equator, where the swine strain appears more dominant than ordinary flu strains, causing more illnesses.
Virus hunters are studying its spread and are on the lookout for any gene-swapping activity between the swine strain and a seasonal H1N1 virus already resistant to Tamiflu, a leading antiviral drug.
Most U.S. swine flu cases have been so mild they have not required treatment, but Schuchat said federal officials still do not know why the virus causes mild illnesses in some people and lethal infections in others.
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