Seasonal influenza is associated with high morbidity and mortality especially in high-risk populations, and is known to cause social and economic loss. However, the disease burden of seasonal influenza in the Korean population is yet to be quantified. While over 10 million Koreans are vaccinated annually with the seasonal influenza vaccine, studies on the effectiveness of the vaccine are lacking. The objectives of this study are 1) evaluate the disease burden of seasonal influenza and 2) evaluate the effectiveness of the seasonal influenza vaccine in the Korean population.