1. Centro de Investigación Biomédica en Red de Epidemiología y Salud Pública (CIBERESP; Network of Biomedical Research Centers Epidemiology and Public Health), Spain
2. Instituto de Salud Pública de Navarra (Public Health Institute of Navarre), Pamplona, Spain
3. Complejo Hospitalario de Navarra (Hospital complex of Navarre), Pamplona, Spain
4. Clínica Universidad de Navarra (University Clinic of Navarre), Pamplona, Spain
5. National Centre of Microbiology (World Health Organization National Influenza Centre - Madrid), Instituto de Salud Carlos III, Majadahonda, Spain
6. Primary Healthcare Directorate, Navarre Health Service, Pamplona, Spain
7. The members of these networks are listed at the end of the article