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By David Brown, Washington Post
Published February 10, 2008
WASHINGTON - Seasonal influenza is spreading widely throughout the United States, with nearly half the cases caused by strains of the virus that are not directly covered by this year's flu vaccine.
Whether the winter will end up being worse than usual remains to be seen. Flu mortality in adults has been higher than in the last two years, but deaths in children - an important marker of severity - have been rare.
Nevertheless, this winter is likely to be one of the few times that public health experts lose the bet they make each year when they devise the formula for the flu vaccine, eight months before the virus starts circulating in the fall. Experts must decide on the formulation then because of the time it takes to produce mass quantities of the vaccine.
"Most years, the prediction is very good," said Joseph Bresee, an influenza epidemiologist at the Centers for Disease Control and Prevention. "In 16 of the last 19 years, we have had a well-matched vaccine."
But probably not this time.
Each year, the vaccine contains representatives of the three huge families of flu virus that are currently circulating. They are two main types of influenza A, H1N1 and H3N2, and influenza B.
A new strain of H3N2 virus was identified in Brisbane, Australia, last February, a few weeks after the components of this winter's vaccine were chosen. But it was too late to substitute "Brisbane/10," the short version of its name, for the H3N2 strain, called "Wisconsin."
From the start of flu season until the beginning of February, 34 percent of flu viruses taken from patients around the country were Brisbane strains.
At the same time, a strain of influenza B called "Yamagata," which is significantly different from the "Victoria" B strain in the vaccine, was taking off.
Together, the Brisbane and Yamagata strains are accounting for 48 percent of all flu samples this winter.
That does not mean the vaccine is of no benefit. The immunity conferred by the Wisconsin strain may protect somewhat against its Brisbane descendant; a vaccinated person may have milder symptoms. But the vaccine is unlikely to prevent infection altogether in lots of people.
[Last modified February 10, 2008, 01:34:08]