Influenza is an infectious disease with person-to-person transmission that characteristically occurs as an epidemic affecting the whole population . The influenza virus has been categorized into types A, B and C. However influenza C is a mild disease without seasonality and is therefore not considered in influenza epidemics. One remarkable feature of the influenza A and B viruses is the frequency of changes in antigenicity. Alterations in the antigenic structure of the virus leads to infection by variants to which the population has little or no immunity.