Influenza causes seasonal illness characterized by fever, myalgia and respiratory symptoms which can lead to hospitalization and death. Although it is a vaccine preventable disease, influenza contributes directly and indirectly to a large number of hospitalizations and outpatient visits. More specifically, influenza causes every year approximately 12,000 hospitalizations and 3,500 deaths in Canada, of which 90% occur in people 65 years and older. Recently, numerous studies have investigated the impact of repeated vaccination on its effectiveness reporting a large variety of results.