Feb. 18, 2009 -- When it comes to predicting type B flu, the experts can't win for losing.
There are two major lineages of type B flu: B Victoria and B Yamagata. Last year, experts put the Victoria lineage in the flu vaccine -- and 98% of type B flu bugs turned out to be from the Yamagata lineage.
So this year, they made the vaccine protective against Yamagata, but now two-thirds of the type B flu bugs in circulation are from the Victoria lineage.
"Limited to no protection may be expected when the vaccine and circulating virus strains are so different as to be from different lineages, as seen with the two lineages of influenza B viruses," the CDC says in its weekly flu report.
Fortunately, the flu vaccine is a perfect match for the two type A flu bugs now in circulation. This early in the flu season, it's too soon to tell which strains will predominate. But last year, about 30% of flu bugs were type B.
The Advisory Committee on Immunization Practices recommends U.S. flu policy, although the flu vaccine ingredients are officially chosen by the FDA. This week, an FDA panel voted to change the type B component of next year's flu vaccine type B component. And on the agenda for next week's ACIP meeting: a discussion of whether to put both B lineages in future flu vaccines.
Flu Season Arrives
The frustrating news about the vaccine's type B component comes just as flu season is heating up in the U.S.
The number of people seeing a doctor for flu-like illnesses is higher than average and is trending sharply upward.
As of Feb. 7, the date of the latest surveillance report, 16 states had widespread flu activity (meaning flu outbreaks in areas totaling more than half the state's population). Another 16 states had regional flu activity (meaning outbreaks in areas totaling less than half the state's population).
Last year, flu season peaked in mid-February and hung around until mid-April. That was longer than the previous three years, in which flu season peaked in mid-February but ended in early March.
But it's anybody's guess what will happen this year, as one season's flu activity does not necessarily predict the next.