Last Year's Flu Shot Imperfect But Effective

CDC finds it gave more protection than experts had thought

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HealthDay Reporter

THURSDAY, Aug. 12, 2004 (HealthDayNews) -- Last year's influenza vaccine was far from a perfect match against the virus that sickened people, but it offered more protection from the illness than experts had previously thought.

In very young children, the shot was found to be 25 percent to 49 percent effective in preventing influenza-like illness, which is a suspected case of flu that wasn't confirmed in a laboratory. In adults, it was 38 percent to 52 percent effective in protecting against laboratory-confirmed flu.

Experts had been concerned about last year's vaccine because the particular strains circulating during the 2003-2004 flu season did not exactly match those included in the vaccine. Adding to the concern was a smaller, preliminary study last winter finding it failed to adequately protect health-care workers.

"The more similar the circulating virus is to the virus included in the vaccine, the better the performance in the vaccine," said Dr. Carolyn Bridges, co-author of an article appearing in the Aug. 13 issue of Morbidity and Mortality Weekly Report, published by the U.S. Centers for Disease Control and Prevention. Bridges is a medical epidemiologist at the CDC's National Immunization Program.

When the vaccine is a match, it is expected to provide 70 percent to 90 percent immunity in otherwise healthy young adults.

This year's vaccine should be a better match with circulating flu strains. "I don't know of any indications of concern about the match for this coming year," Bridges said. She added a cautionary note, however, that "influenza is always a moving target, which makes it so interesting and also somewhat frustrating."

Nailing the best vaccine is part epidemiology and part guesswork. Typically, it is prepared months ahead of time so it's ready to administer in time for the flu season. In the United States, experts take note of what strains of virus are circulating in other parts of the world, notably Asia, and anticipate its arrival.

But last season, a new strain of flu began circulating before health officials had a chance to prepare for it. Making matters worse, the flu hit early, and hit Colorado particularly hard. Reports of illnesses and deaths helped create a run on the vaccine, prompting a shortage.

The CDC and other experts emphasized that the protection seen last year, even when the match was not optimal, was still substantial. A vaccine, even an imperfect one, is still the way to go, they said.

"You do get some protection regardless of whether the match is exact," said Dr. Marc Siegel, a clinical associate professor of medicine at New York University School of Medicine. "People who are in vulnerable groups, especially, should get their flu vaccine regardless of what the current thinking is about the match."

The current conclusions reflects findings from two studies, one in children and one in adults, focusing on the period between Nov. 19 and Dec. 7, 2003, the peak influenza period in the Denver area.

The first study involved 5,139 children aged 6 to 23 months of age. By Nov. 19, 7 percent were fully and 20 percent were partially vaccinated against influenza (two shots are recommended for the youngest recipients). By Dec. 7, 15 percent and 27 percent were fully or partially vaccinated. The vaccine was 25 percent to 49 percent effective in preventing influenza-like illness.

The second study looked at about 1,800 adults 50 to 64 years of age. The vaccine turned out to be 52 percent effective in preventing laboratory-confirmed influenza among otherwise healthy adults. It was also 38 percent effective in adults with one or more chronic medical conditions.

Bridges said several factors accounted for the differences between the earlier study on health-care workers and the newest ones. Notably, the surveys in adults and children were much larger.

This is also not the first time that two reports from the same year have been contradictory. In 1997, Bridges said, the vaccine did not match exactly, and studies done at different times suggested different effectiveness rates. "It's something that has happened in the past, and that's why we initiated several other studies," Bridges said.

More information

The U.S. Centers for Disease Control and Prevention has more on the flu and the flu vaccine.

SOURCES: Carolyn Bridges, M.D., medical epidemiologist, National Immunization Program, U.S. Centers for Disease Control and Prevention, Atlanta; Marc Siegel, M.D., clinical associate professor, medicine, New York University School of Medicine, New York City; Aug. 13, 2004, Morbidity and Mortality Weekly Report

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