Flu vaccine effectiveness lower than usual

By Will Dunham

WASHINGTON (Reuters) - This season's influenza vaccine in the United States appears to be much less effective than usual, health officials said on Thursday, with the strains of the virus in circulation defying what vaccine designers had anticipated.

Based on findings from a community in central Wisconsin that served as a test case, people who were vaccinated were 44 percent less likely to get influenza than unvaccinated people, the U.S. Centers for Disease Control and Prevention said. That compares to typical vaccine effectiveness of around 70 percent or higher, CDC officials said.

The CDC and the World Health Organization have recommended a complete reformulation of next year's flu vaccine because of the poor match during the 2007-2008 flu season, which officials have described as moderately severe.

This year's study confirmed suspicions that two of the three strains of flu most likely to cause disease had "drifted," or evolved, away from the cocktail used in the vaccine.

Flu infects from 5 percent to 20 percent of the U.S. population each year and kills an estimated 36,000 Americans in an average year, most of them elderly. It also can kill young children. This season, the CDC counted 66 child deaths from flu.

The vaccine is designed to protect against three influenza strains -- two from Type A, an H1N1 and an H3N2 version, and one for Type B. "This season, we had a less-than-optimal match in two of the three vaccine components," Dr. Dan Jernigan of the CDC's Influenza Division told reporters.

It is tricky to compare vaccine effectiveness year to year because the CDC has used various tools and different populations to track it but it appears this year's vaccine effectiveness may be the lowest since the 1997-1998 flu season. The vaccine must be reformulated every year because the viruses mutate so much.

The Wisconsin study did not include MedImmune's FluMist vaccine, which is formulated differently from the shots and is squirted up the nose.

BAD MATCHES

This flu season was only the fourth in the past 20 years in which the viruses targeted in the vaccine were not good matches for viruses actually circulating among people, Jernigan said.

"For the 1997-98 season, the vaccine effectiveness was effectively zero," Jernigan said.

Nevertheless, CDC officials said the current vaccine still provided some protection, although none against influenza B viruses. Most influenza B viruses circulating in the United States this year were in a different lineage than the strain included in the vaccine, the CDC said.

The CDC also found that about 8 percent of all H1N1 viruses that made people sick had some resistance to the flu drug Tamiflu, made by Roche AG and Gilead Sciences under the generic name oseltamivir.

Health officials decide on the viruses to be included in the vaccine months before the flu season begins, predicting what strains will be circulating that year.

Jernigan said flu activity in the United States peaked in mid-February, but the virus still could circulate into May.

"At this point, the 2007-2008 season appears to be most similar to the 2003-2004 season. And that season was characterized as moderately severe," Jernigan said.

Dr. Jeanne Santoli of the CDC's Immunization Services Division said a record 140 million doses were provided by six manufacturers this flu season.

"Looking ahead, we anticipate that next season's vaccine supply will be similar or somewhat increased to what we have available this season," Santoli said.

The CDC is now recommending all children age six months to 18 years should be vaccinated each year against flu. The previous advice had ended at age 5, and the recommendations add about 30 million children to those who should be vaccinated.

(Editing by Maggie Fox and Bill Trott)