Avian influenza represents one of the greatest concerns for public health that has emerged in recent times. Highly pathogenic avian influenza viruses belonging to the H5N1 subtype are endemic in Asia and are spreading in Europe and Africa. Vaccination is now considered a tool to support eradication efforts, provided it is appropriately managed. This study was carried out to establish the degree of clinical protection and reduction of viral shedding induced by a high-specification, commercially available avian influenza vaccine of a different lineage and containing a strain with a heterologous neuraminidase (H5N9 subtype) to the challenge virus isolate A/chicken/Yamaguchi/7/2004 (H5N1 subtype).