2010-2011 Trivalent Influenza Vaccine with All Three Vaccine Strains Was Moderately Effective

U.S. investigators assessed vaccine effectiveness during the 2010-2011 season in an attempt to characterize the relationship between antigenic characteristics of circulating viruses and effectiveness of seasonal influenza vaccines. They employed a case-control study design, capturing vaccination histories both for subjects with acute respiratory illness confirmed by positive real-time reverse transcription polymerase chain reaction (PCR) for influenza, and influenza test-negative controls.

Subjects in communities in four states with acute respiratory illness of ≤7 days duration were enrolled. History of immunization with the 2010-2011 vaccine was ascertained from vaccine registries or medical records. Vaccine effectiveness was estimated using logistic regression models adjusted for study community, age, race, insurance status, enrollment site and presence of a high-risk medical condition

A total of 373 cases of influenza A/H1N1, 334 cases of influenza A/H3N2 and 333 cases of influenza B were identified, and their vaccination histories were compared against 3,717 influenza-negative controls. Overall adjusted vaccine effectiveness was 60% (95% confidence interval [CI], 53% to 66%). Age-specific vaccine effectiveness estimates ranged from 69% (95% CI, 56% to 77%) in children aged 6 months to 8 years, to 38% (95% CI, -16% to 67%) in adults aged ≥65 years. It was judged that influenza vaccines were moderately effective in preventing medically attended influenza during this 2010-2011 season in which all three flu vaccine strains were antigenically similar to circulating viruses.