An inactivated, alum-adjuvanted, whole-virion H5N1 vaccine had been evaluated previously. Hemagglutination inhibition ( HI) assays showed that the antibody levels declined significantly, with 4.8%-20.8% and 0%-18.8% of participants retaining seroprotection (HI titer >= 1:40) 6 and 12 months after the second dose, respectively. A third dose of the same vaccine given 12 months after the second dose significantly boosted immune responses. Thirty days after the third dose in the 1.25-, 2.5-, 5-, and 10-mu g dose groups, 29.4%, 31.3%, 78.6%, and 90.0% of participants had HI titers >= 1: 40, and52.9%, 81.2%, 92.9%, and 100% of participants had microneutralization titers >= 1: 40, respectively. Both the 5-mu g and 10-mu g doses met European Union criteria. (ClinicalTrials.gov identifier: NCT00660257.)