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

Traveler Danny Manzon of Jacksonville, Fla., gets a flu shot from Stephanie Parks at Hartsfield-Jackson Atlanta International Airport. (By John Bazemore -- Associated Press)

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 44 percent of all flu samples this winter -- and neither is in the vaccine.

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.

A study done by the Defense Department last year after Brisbane emerged found that it was 52 percent effective in preventing infection. That is much lower than the 70 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 J. Cox, the chief flu virologist at CDC.

The agency is doing three studies to determine with precision how much protection the vaccine is providing against the Brisbane strain this winter. Results of the first may be available in a few weeks.

The vaccine's protection against the Yamagata strain 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.

Early this season, most of the virus samples collected from patients were from the H1N1 family and were adequately protected against by the vaccine. There were few H3N2 or influenza B viruses circulating.

Now, however, the new strains of those latter two types have arrived, and they are encountering millions of people who are vulnerable because they have not been vaccinated, or partly vulnerable because the vaccine offers partial protection. As a result, the new strains have taken off.

The portion of each week's samples made up of H3N2 viruses has been rising steadily for more than a month. For the past two weeks, it has exceeded the fraction containing H1N1 viruses, and it has not yet peaked. In four of the nine surveillance regions across the United States, H3N2 viruses are now more common than H1N1 viruses.

All of which suggests the flu season may be far from over.

As a defense, CDC officials say vaccination, even now, may be of some benefit. They say antiviral drugs can both prevent infections and lessen their severity, and are especially useful in people at high risk for flu complications because of other illnesses. They also urge people to cover their coughs, wash their hands often and stay home if they are sick.

Meanwhile, influenza virologists from around the world will gather in Geneva this week to decide the formula for next year's vaccine.