Published on: 02/11/08
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 to allow time to produce mass quantities of the vaccine.
"Most years, the prediction is very good," said Joseph Bresee, an influenza epidemiologist at the Atlanta-based 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 each of these lineages are constantly changing through mutation. Inevitably, one appears that is different enough that a person protected against them is not protected against the new variant.
A version of this scenario 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.
But it was too late to substitute "Brisbane/10" for the H3N2 strain, called "Wisconsin," that had been in the vaccine since the 2006-07 season. 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.
About 16 percent of all flu samples this winter are influenza B, and of them 93 percent are Yamagata.
Together, the Brisbane and Yamagata strains are accounting for 48 percent of all flu samples this winter —- and neither is in the vaccine.
That does not mean the vaccine is without 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.
A study done by the Department of Defense last year after Brisbane emerged found that it was 52 percent effective in preventing infection.
That is much lower than the 70 percent to 90 percent protection provided by a well-matched vaccine given to healthy, young adults. But it is not useless, either.
"It wouldn't be optimal, but there should be a measure of protection, based on our past experience," said Nancy Cox, the chief flu virologist at CDC.
Protection against the Yamagata strain by the vaccine is probably also poor. The best evidence that Yamagata is not well covered by the vaccine is that it is just about the only strain of influenza B around.
"That probably indicates that the effectiveness of the vaccine may be less than ideal," Bresee said.
Influenza virologists from around the world will gather in Geneva, Switzerland, this week to decide the formula for next year's vaccine.