1. Department of Preventive Medicine and Public Health, University of Valladolid, 47005 Valladolid, Spain
2. Preventive Medicine and Public Health Service, Hospital Clínico Universitario de Valladolid, 47003 Valladolid, Spain
3. National Influenza Centre, Edificio Rondilla, Hospital Clínico Universitario de Valladolid, 47009 Valladolid, Spain
4. SALBIS Research Group, Faculty of Health Sciences, Ponferrada Campus, Universidad de León, 24401 Ponferrada, Spain
5. Microbiology Service, Hospital Universitario Río Hortega, 47012 Valladolid, Spain
6. Infectious Diseases Unit, Hospital Clínico Universitario, 47003 Valladolid, Spain
7. Dermatology Service, Complejo Asistencial Universitario de León, 24008 León, Spain