Even though the influenza vaccine used in the US during last winter’s flu season was not optimally matched to the predominant circulating strain of the virus, it nevertheless delivered a substantial health benefit, according to a report from the CDC.

Dr. D. Ritzwoller, at Kaiser Permanente Colorado in Denver, and associates conducted studies of children and adults to assess the effectiveness of the 2003-2004 influenza vaccine.

From electronic medical records, the investigators identified 5139 children between 6 and 23 months old enrolled in the HMO between October 1 and December 31, 2003. By December 7, 15 percent of the children were fully vaccinated with two flu shots.

The likelihood of an influenza-like illness in vaccinated children was 75 percent of that for unvaccinated children, meaning that the vaccine had an effectiveness of 25 percent.

The vaccine was 49 percent effective in preventing pneumonia and influenza, the team reports in the CDC’s Morbidity and Mortality Weekly Report.

Children who received partial immunization did not have a significantly reduced risk of influenza-like illness, pneumonia or influenza compared with unvaccinated children, the investigators found.

For the adult study, the research team identified and contacted 330 patients between 50 and 64 years old who developed influenza between November 1 and December 31. These individuals were matched with 1055 “control” subjects who did not catch the flu.

Ritzwoller’s group estimates the vaccine was 52 percent effective in preventing influenza in subjects without a high-risk medical condition, and 38 percent effective for those with medical conditions associated with an increased risk for influenza-related complications.

The authors note that when vaccine and circulating strains are well-matched, the expected vaccine effectiveness among healthy adults is 70 to 90 percent.

Overall, these figures support “recommendations to continue influenza vaccination efforts despite a suboptimal match” between the predominant circulating flu virus and the strains in the available vaccine, Ritzwoller’s team concludes.

SOURCE: Morbidity and Mortality Weekly Report, August 13, 2004.