2007-2008 Influenza Vaccine Reasonably Effective Despite Mismatch

The trivalent inactivated influenza vaccine used during the 2007-2008 influenza season in the US was 44% effective in preventing infection, even though the match between two of the vaccine strains and circulating strains was "suboptimal," according to an interim, within-season analysis.

The results of the study by researchers from the Centers for Disease Control and Prevention reinforce the message that laboratory testing for antigenic match alone is not sufficient to assess vaccine efficacy, and clinical data must be considered as well. The findings are based on an analysis of data for patients living around Marshfield, Wisconsin, an area where nearly all residents receive care from Marshfield Clinic healthcare providers.

Vaccine efficacy in patients for whom the Advisory Committee on Immunization Practices recommends immunization (young children, older adults, and persons with chronic medical conditions) was 34%. The efficacy in individuals falling outside these parameters was higher -- 54%, according to the findings released Thursday in the Morbidity and Mortality Weekly Report.

In preventing medically attended influenza A infections, the vaccine was 58% effective. No efficacy was noted in preventing medically attended influenza B infections.

Despite the suboptimal antigenic match, the findings suggest that immunization with the 2007-2008 vaccine "provided substantial protection against medically attended acute respiratory illness in this study population," the report concludes.

"Intraseason estimates of vaccine efficacy, such as those from this analysis, might be useful to public health authorities and medical practitioners in their communications about the benefits of vaccination, especially late in the influenza season."

Mor Mortal Wkly Rep CDC Surveill Summ 2008;57:393-398.

Reviewed By Ramaz Mitaishvili, MD

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