Vaccine compositions, useful for eliciting an immune response in subjects which is protective against influenza type A virus, comprise influenza type A virus antigen and a bacterial sialidase. An intranasal vaccine against highly pathogenic subtype H5N1 virus, for use in the treatment of poultry, preferably comprises inactivated H5N1 antigen, sialidase from Clostridium perfringens A strain 107 and chitosan. The use of bacterial sialidase to potentiate influenza virus antigen vaccine is also disclosed.
