Primary Outcome Measures:
- HI antibody titer for younger adults age 18 to 64 years and older adults age >64 years [ Time Frame: At 21 days after the second dose of GSK 1557484A vaccine ]
- The occurrence of specifically-solicited local and general signs and symptoms, and overall per subject considering both post-immunization periods [ Time Frame: During a 7-day follow-up period (i.e., day of vaccination and 6 subsequent days) after each vaccine administration ]
- The occurrence of all unsolicited adverse events [ Time Frame: During a 21-day follow-up period for each vaccine administration, as well as overall (Day 0 through Day 84) ]
- The occurrence of serious adverse events and medically-attended events [ Time Frame: Day 0 through Day 182. ]
Secondary Outcome Measures:
- HI antibody titer for younger adults age 18 to 60 years and older adults age >60 years [ Time Frame: At 21 days after the second dose of GSK 1557484A vaccine ]
- HI antibody titer for younger adults age 18 to 64 years and older adults age >64 years. [ Time Frame: At 6 months after the first dose of GSK 1557484A vaccine ]
- Vaccine-homologous virus and drift variant H5N1 virus antibody responses, as assessed by microneutralization assays, in subjects receiving 2 doses of study vaccine.
A Phase 3, observer-blind, placebo-controlled, multi-center study designed to evaluate the safety and immunogenicity of an investigational vaccination regimen in adults aged 18 years or older. A total of 4440 subjects are to be enrolled in the study at 35 to 50 study centers in the United States and Canada, of whom ~3330 are to receive study vaccine and ~1110 are to receive placebo. Subjects will attend formal study center visits for study assessments on Days 0, 21, 42, and 182. In addition, a telephone visit will be conducted on Day 84.