Background: The reduced immunogenicity of the H5 hemagglutinin (HA), compared to seasonal HA serotypes, has stimulated searches for effective adjuvants to improve H5 vaccine efficacy. This study examined the immunogenicity and protective efficacy in ferrets immunized with a split-virion H5N1 vaccine combined with Advax™, a novel delta inulin-based polysaccharide adjuvant technology that has previously demonstrated ability to augment humoral and cellular immunity to co-administered antigens. Methods: Ferrets were vaccinated twice 21 days apart with 7.5μg or 22.5μg of a split-virion preparation of A/Vietnam/1203/2004 with or without adjuvant. An additional group received just one immunization with 22.5μg HA plus adjuvant. Serum antibodies were measured by hemagglutination inhibition and microneutralization assays. Vaccinated animals were challenged intranasally 21 days after the last immunization with 10 6 EID 50 of the homologous strain. Morbidity was assessed by observed behavior, weight loss, temperature, cytopenias, histopathology, and viral load. Results: No serum neutralization antibody was detected after two immunizations with unadjuvanted vaccine. Two immunizations with high or low dose adjuvanted vaccine stimulated high neutralizing antibody titers. Survival was 100% in all groups receiving adjuvanted-vaccine including the single dose group, compared to 67% survival with unadjuvanted vaccine, and 0% survival in saline or adjuvant-alone controls. Minimal morbidity was seen in all animals receiving adjuvanted vaccine, and was limited to rhinorrhea and mild thrombocytopenia, without fever, weight loss, or reduced activity. H5N1 virus was cleared from the nasal wash by day 4 post-challenge only in animals receiving adjuvanted vaccine which also prevented viral invasion of the brain in most animals. Conclusions: In this initial study, Advax™ adjuvant formulations improved the protective efficacy of a split-virion H5N1vaccine as measured by significantly enhanced immunogenicity, survival, and reduced morbidity. © 2011 Elsevier Ltd.