Seasonal influenza is characterized by annual outbreaks in temperate countries in the sense that there is almost invariably a marked seasonal increase in case rate during the winter months. However, it cannot be said to be strictly periodic in the sense that successive outbreaks are comparable with each other in magnitude and duration. In this sense, seasonal influenza has no clear periodicity and contrasts with the behaviour of diseases such as measles, which has a pronounced biennial structure in pre-immunization period [5] .