The mean length of a typical influenza season in Europe based on the peak activity levels of ILI/ARI was 15.6 weeks. It is a conservative estimate, because it does not include the period of increased influenza activity. If this period is also taken into account the average European influenza season lasts about 4.5 months. This is a important finding as it highlights the fact that influenza activity occurs for a long period of time in Europe each season. It also highlights the need to present country-specific data, in order to get insight into the diversity of activity [16] .