1. European Centre for Disease Prevention and Control, Stockholm, Sweden
2. World Health Organisation Regional Office for Europe, Copenhagen, Denmark
3. Robert Koch Institute, Berlin, Germany
4. Santé Publique France – Direction des maladies infectieuses, Saint-Maurice, France
5. Centre national de référence Virus des infections respiratoires, dont la grippe, Institut Pasteur, Paris, France
6. Centre national de référence Virus des infections respiratoires, dont la grippe, Hospices civils de Lyon, Lyon, France
7. Istituto Superiore di Sanita, Rome, Italy
8. Istituto Nazionale Malattie Infettive Lazzaro Spallanzani, Rome, Italy
9. Coordination Centre for Health Alerts and Emergencies. Spanish Ministry of Health, Madrid, Spain
10. Servicio de Epidemiología, Dirección General de Salut Pública, Islas Baleares, Spain
11. Federal Service for Surveillance on Consumer Rights Protection and Human Well-being (Rospotrebnadzor), Moscow, Russia
12. Department of Infectious Disease Prevention and Control, Common Community Commission, Brussels-Capital Region, Brussels, Belgium
13. Chief Physician, Infection control unit, Lapland Hospital District, Rovaniemi, Finland
14. County Medical Officer, Jönköping Region, Jönköping, Sweden
15. Santé publique France – Direction des régions, Cellule régionale Nouvelle Aquitaine, Bordeaux, France
16. Santé publique France – Direction des régions, Cellule régionale Ile-de-France, Paris, France
17. Santé publique France – Direction des régions, Cellule régionale Auvergne-Rhône-Alpes, Lyon, France
18. National Centre for Microbiology, WHO-National Influenza Centre, Institute of Health Carlos III. Madrid, Spain
19. National Centre of Epidemiology, CIBERESP, Institute of Health Carlos III. Madrid, Spain
20. Dirección General de Salut Pública, Islas Baleares, Spain
21. State Research Center of Virology and Biotechnology “Vector”, Rospotrebnadzor, Moscow, Russia
22. St. Pierre Hospital, Brussels, Belgium
23. Laboratory of Clinical Virology, Department of Microbiology and Immunology, Rega Institute, KU Leuven - University of Leuven, Leuven, Belgium
24. Epidemiology of infectious diseases, Sciensano, Brussels, Belgium
25. Expert Microbiology Unit, Department of Health Security, Finnish Institute for Health and Welfare (THL), Helsinki, Finland
26. Infectious Disease Control and Vaccinations Unit, Department of Health Security, Finnish Institute for Health and Welfare (THL), Helsinki, Finland
27. Public Health Agency of Sweden, Solna, Sweden
28. Jönköping Region, Jönköping, Sweden
29. These authors have contributed equally to the manuscript