Molecular mechanisms of variation in influenza viruses

Influenza is unique among the viruses in its capacity to vary. Because of antigenic variation, it has proved impossible to control influenza by vaccination, and variation in virulence, host range and transmissibility influence the spread and severity of influenza epidemics. This variation is caused by sequence changes in the genes of the virus and this review summarizes recent information on the structure of the genes and their products, the way in which these vary and the effects of the changes on the biological activities of the virus.