1. MR and EK 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. Public Health England, London, United Kingdom
6. Department of Virus and Microbiological Special diagnostics, National Influenza Center, Statens Serum Institut, Copenhagen, Denmark
7. Inmaculada Casas National Centre for Microbiology, National Influenza Reference Laboratory, World Health Organization National Influenza Centre, Institute of Health Carlos III
8. Health Protection Scotland, Glasgow, United Kingdom