European Centre for Disease Prevention and Control, Carlos III Institute of Health - National Center for Epidemiology, Public Health Agency of Sweden, Carlos III Institute of Health - Communicable Diseases Department, National Institute of Public Health Romania-National Centre for Communicable Diseases Surveillance and Control, Sciensano - Department of Epidemiology and Public Health, Sciensano, National Institute of Public Health (Czech Republic) - Department of Infectious Diseases Epidemiology, National Institute of Public Health (Czech Republic) - Department of Infectious Diseases Epidemiology, Public Health Agency of Sweden, National Health Institute Doutor Ricardo Jorge (INSA) - National Influenza Reference Laboratory, Medical University of Vienna - Department of Virology, Superintendence of Public Health - Infectious Disease Prevention and Control Unit, Superintendence of Public Health - Infectious Disease Prevention and Control Unit, National Institute for Public Health and the Environment (RIVM), National Institute for Public Health and the Environment (RIVM), Health Protection Surveillance Centre (HPSC), Health Protection Surveillance Centre (HPSC) and Government of the United States of America - Centers for Disease Control and Prevention (CDC)