Preparation for an H5N1 influenza pandemic in humans may involve priming the population with a vaccine produced from an existing, available H5N1 strain. We have used a mouse challenge model to compare the immunogenicity and efficacy of inactivated, Vero cell-derived, whole virus H5N1 vaccines in single immunization and homologous or heterologous prime-boost regimes. A single immunization was sufficient to protect against a lethal challenge with strains from matched and unmatched H5N1 clades. Homologous and heterologous prime-boost regimes induced cross-neutralizing antibodies and cross-protection against representative viruses of H5N1 clade 1, clade 2.1, clade 2.2 and clade 2.3. Moreover, the results indicate that heterologous prime-boost immunization regimes might broaden the specificity of the anti-H5N1 antibody response.