The present invention is based on the surprising finding that H5 protein of clade 1 H5N1 induces, in particular by a single-shot vaccination, a cross-clade protective immune response to influenza viruses with H5N1 HA. In one aspect, the invention is thus directed to H5 protein of clade 1 H5N1 virus for use in a method of treating or preventing infections with H5N1 virus of a different clade, wherein the H5N1 virus of a different clade comprises a particular polynucleotide and/or H5 protein.
