U.S. Flu vaccine effectiveness lower than usual

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U.S. Flu vaccine effectiveness lower than usual

Last Updated: 2008-04-18 12:01:42 -0400 (Reuters Health)

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 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.