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Influenza viruses are adept in human populations. Indeed, they have the capacity to evade the immune system through mechanisms of mutations (antigenic drift) and major variations in surface protein expression (antigenic shift). When a major change occurs, the risk of a human pandemic arises. Three influenza pandemics occurred during the 20th century, the most serious being the Spanish influenza. The last pandemic of the past century occurred in 1968, and the responsible virus infected an estimated 1-3 million people throughout the world. The first pandemic of the present century occurred in 2009 and was sustained by a H1N1 strain (A/California/07/09). In 1997, a novel avian influenza virus, H5N1, first infected humans in China. Since its emergence, the H5N1 virus has spread from Asia to Europe and Africa, resulting in the infection of millions of poultry and wild birds. So far, 522 human cases and 322 deaths have been reported by the WHO. Many studies have therefore been performed to obtain efficacious and safe H5N1 vaccines. One of these is Aflunov(®). Aflunov is a prepandemic monovalent A/H5N1 influenza vaccine adjuvanted with MF59 produced by Novartis Vaccines and Diagnostics. In nonclinical studies conducted in rabbits, Aflunov proved to be well-tolerated, did not cause maternal or embryo-fetal toxicity, was not teratogenic, and had no effects on postnatal development. In clinical studies, Aflunov proved safe and well-tolerated in infants, children, adolescents, adults and the elderly. In the same subjects, the vaccine elicited robust immunogenicity against both homologous (A/Vietnam/1194/2004 clade 1) and heterologous viral strains (for instance, A/Indonesia/05/2005 or A/Turkey/15/2006) and induced immunologic memory. Thus, in 2010, the CHMP issued a positive opinion on Aflunov and in January 2011 Aflunov was given marketing authorization. This vaccine could be very useful in the event of adaptation of the H5N1 virus to humans, which could cause a new pandemic.