Evaluation of the safety and immunogenicity of a booster (third) dose of inactivated subvirion H5N1 influenza vaccine in humans

Previously, we evaluated 2 doses of H5N1 influenza vaccine in persons 18-64 years of age (placebo and 7.5-, 15-, 45-, or 90-microg doses), separated by 28 days. In this study, 337 participants received a third dose, 6 months thereafter. Microneutralization (MN) and hemagglutination-inhibition geometric mean titers (GMTs) of antibody declined before the third dose. Twenty-eight days after the third dose, 78%, 67%, 43%, and 31% of recipients in the 90-, 45-, 15-, and 7.5-mug-dose groups had a MN GMT > or =1:40, respectively. Five months later, MN GMTs were significantly greater than those after the second dose. (Trial registration: Clinical Trials.gov identifier NCT00240968 .).