National Influenza Centre, Université Libre de Bruxelles (ULB) - Centre for Environmental Health and Occupational Health, Sciensano - Department of Epidemiology and Public Health, AZ Sint-Jan - Medical Microbiology, Université Libre de Bruxelles (ULB) - Centre for Environmental Health and Occupational Health, Universitair Ziekenhuis Brussel - Internal Medicine-Infectious Diseases, Universitair Ziekenhuis Brussel - Pediatric Pulmonary and Infectious Diseases, Grand Hôpital de Charleroi - Microbiology, Grand Hôpital de Charleroi - General Internal Medicine and Infectiology, Jessa Ziekenhuis - Clinical Laboratory, Jessa Ziekenhuis - Infection Control, Centre Hospitalier Universitaire UCL Namur, Centre Hospitalier Universitaire UCL Namur, Centre Hospitalier Universitaire St-Pierre, National Influenza Centre, Catholic University of Louvain (UCL) - Center for Operations Research and Econometrics (CORE), National Influenza Centre and National Influenza Centre