U.S. flu season worsens as new vaccines ordered

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Influenza is widespread in 49 states, and this year's epidemic has killed at least 22 children, the U.S. Centers for Disease Control and Prevention reported on Friday.

On Thursday the U.S. Food and Drug Administration agreed to completely change next year's flu vaccine mix because all three strains included in the flu shot cocktail have mutated. But companies have a head start on working with two of the three, the CDC's Dr. Nancy Cox said.

FDA advisers agreed with the World Health Organization recommendations made last week on changing the vaccine to match the drifting flu viruses.

"In brief, seasonal influenza activity has increased during the past week," Cox told reporters in a telephone briefing.

Flu has killed 22 children so far this season, Cox said. She did not have details on any of the cases.

Cox said the CDC was watching the epidemic and asking state health departments to collect data on who gets sick, whether the were vaccinated, and whether influenza drugs were effective in fighting the infections.

Several European countries have reported that people are becoming infected with strains that resist the effects of Tamiflu, the antiviral drug made by Roche AG and Gilead Sciences.

Cox said the CDC had seen little evidence that flu was resisting Tamiflu in the United States. "We have actually seen antiviral resistance only sporadically in eight states," she said.

And there are reports that some people who have become ill with confirmed flu did get vaccinated.

The flu was also found in Florida, but the CDC said it had not become widespread.

FRESH COCKTAILS

Flu vaccines contain a mixture of two influenza "A" strains, which currently are types known by the shorthand of H3N2 and H1N1, and a "B" strain. These mutate or "drift" a little each year, which is why the vaccine must be formulated freshly each season.

This happens in the late winter or early spring in each hemisphere -- February in the Northern Hemisphere and September in the Southern.

"We would say based on our laboratory data that the match is not optimal both for the H3N2 component of the vaccine and also for the influenza B component," Cox told reporters in a telephone briefing.

"In Europe they have primarily had influenza A H1N1. Data generated in Europe indicate that strains circulating there are not so well-matched with the vaccine."