Besides the personal protection, influenza vaccination especially needs to prevent transmission of influenza between GPs and their patients. The question remains if the vaccine is adding substantial benefit to the natural acquired immunity of GPs. Doubts are raised if an inactivated vaccine, which elicit especially humoral immune response, can give enough protection against virus replication and subclinical influenza infections. Until now no effectiveness studies of influenza vaccination were performed among GPs. This study will assess the effectiveness of an inactivated influenza vaccine in GPs against clinical respiratory tract infections and more particular against influenza cases with influenza positive nose and throat swabs (diagnosed by RT-PCR).