World Health Organization (WHO), Sciensano - Department of Epidemiology and Public Health, Université Libre de Bruxelles (ULB) - Centre for Environmental Health and Occupational Health, AZ Sint-Jan - Medical Microbiology, 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, Sciensano - Department of Epidemiology and Public Health, National Influenza Centre, Sciensano, Sciensano - Department of Epidemiology and Public Health and National Influenza Centre