This Season's Flu Strains Are Not a Good Match for Vaccine

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 past 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 S. 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.

The viruses in the vaccine are either dead or, in the case of the nasal-spray flu vaccine developed four years ago, crippled so they cannot cause illness. What they can do is stimulate the body's immune system to mount a defense, sometimes a lifesaving one, should the virus be encountered.

The viruses in each of these lineages are constantly changing through mutation. Inevitably, one appears that is different enough from its ancestors that a person protected against them, through either previous infection or vaccination, is not protected against the new variant.

Such an emergent virus easily finds victims because almost nobody has immunity against it.

A version of this scenario -- muddied, of course, by real life -- apparently happened twice this year.

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. (Later studies showed it had been around at least since January 2007.) But it was too late to substitute "Brisbane/10" -- the short version of its name -- for the H3N2 strain that had been in the vaccine since the 2006-2007 season, called "Wisconsin."

Even if there had been time, it was not certain the Brisbane strain would take off and spread. It has.