Interim within-season estimate of the effectiveness of trivalent inactivated influenza vaccine--Marshfield, Wisconsin, 2007-08 influenza season.

During clinical trials, the efficacy of vaccination with inactivated influenza vaccines for the prevention of serologically confirmed influenza infection has been estimated as high as 70%-90% among healthier adults. However, the effectiveness of annual influenza vaccination typically is lower during those influenza seasons when a suboptimal match between the vaccine strains and circulating influenza strains is observed. For example, in a 4-year randomized study of influenza vaccine among healthy persons aged 1-65 years, the predominant strain was drifted from the vaccine strain in 2 of the 4 years. Inactivated vaccine effectiveness (VE) against culture-confirmed influenza ranged from 71% to 79% when the vaccine and circulating strains were suboptimally matched to 74% to 79% when the matches were well matched. In contrast, a 2-year study of inactivated influenza vaccine among healthy adults aged 18-64 years found no measurable VE during a year when a poorly matched strain circulated, but found VE of 86% against laboratory-confirmed influenza during the following year when the vaccine and circulating strains were well matched. Although laboratory data on the antigenic characteristics of circulating influenza viruses compared with vaccine strains are available during influenza seasons, estimates of VE usually have not been made until months after the conclusion of the season. This report summarizes interim results of a 2008 case-control study to estimate the effectiveness of trivalent inactivated influenza vaccine for prevention of medically attended, laboratory-confirmed influenza during the 2007-08 influenza season, when most circulating influenza A (H3N2) and B viruses were suboptimally matched to the vaccine strains. Despite the suboptimal match between two of three vaccine strains and circulating influenza strains, overall VE in the study population during January 21-February 8, 2008, was 44%. These findings demonstrate that, in any season, assessment of the clinical effectiveness of influenza vaccines cannot be determined solely by laboratory evaluation of the degree of antigenic match between vaccine and circulation strains.