1. Authors contributed equally to the study and manuscript writing
2. Epiconcept, Paris, France
3. Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, Copenhagen, Denmark
4. National Epidemiology Centre, Institute of Health Carlos III, Madrid, Spain; CIBER de Epidemiología y Salud Pública (CIBERESP), Institute of Health Carlos III, Madrid, Spain
5. Health Protection Scotland, Glasgow, United Kingdom
6. National Centre for Microbiology, National Influenza Reference Laboratory, WHO-National Influenza Centre, Institute of Health Carlos III, Madrid, Spain; CIBER de Epidemiología y Salud Pública (CIBERESP), Institute of Health Carlos III, Madrid, Spain
7. Department of Virus and Microbiological Special diagnostics, National Influenza Center, Statens Serum Institut, Copenhagen, Denmark
8. Public Health England, London, United Kingdom