High-dose Trivalent Influenza Vaccine Compared to Standard Dose Vaccine in Elderly Adults: Safety, Immunogenicity and Relative Efficacy During the 2009-2010 Season

Background: High-dose trivalent influenza vaccine was developed to improve antibody responses to influenza vaccine in the elderly and hence potentially impact favorably on influenza-associated morbidity and mortality in this population.

Methods: A phase IIIb, multicenter, randomized, double-blind, controlled trial was conducted to compare High-Dose (HD) trivalent inactivated influenza vaccine (60μg of hemagglutinin [HA] per strain) to standard dose (SD) vaccine (15μg of HA per strain) in adults ≥65 years of age. Assessments of safety (serious adverse events [SAE]), immunogenicity (hemagglutination inhibition [HAI] titers) and relative efficacy were performed during the 2009-2010 influenza season, which coincided with the H1N1 pandemic.

Results: A total of 9172 participants were enrolled in 99 research centers in the US (6117 and 3055 randomized to the HD and SD groups, respectively). Within 180 days after vaccination, 6.7% and 6.5% of participants in the HD and SD vaccine groups, respectively, experienced at least one SAE, of which 0.4% and 0.3% had a fatal outcome. A total of 0.5% of participants in both groups discontinued the study due to a SAE. Post-vaccination HAI titers and rate of post-vaccination HAI titer ≥1:40 were significantly higher in the HD group. No cases of influenza caused by viral types/subtypes similar to those in the vaccines were observed. All cases genetically or antigenically characterized were classified as similar to influenza A/California/7/2009 (H1N1), the pandemic strain. The vaccine efficacy of HD vaccine relative to SD vaccine against any influenza viral type/subtype was 12.6% (95% CI -140.5; 65.8) in the intent-to-treat analysis.

Conclusion: High-dose trivalent inactivated influenza vaccine is safe and well tolerated and provides superior immune responses compared to standard dose vaccine. Demonstration of a superior vaccine efficacy requires a separate large randomized, controlled trial.