1. Infectious Diseases Epidemiology Unit, National Health Institute, Rome, Italy
2. Viral Diseases and Attenuated Vaccines Unit, National Health Institute, Rome, Italy
3. Department of Health Sciences, University of Genoa, Genoa, Italy
4. Department of Biomedical Sciences and Human Oncology, Hygiene Section, University of Bari, Bari, Italy
5. Regional Reference Centre for Infectious Diseases, Microbiology and Virology Unit, Padua University Hospital, Padua, Italy
6. Infectious Diseases and International Prophylaxis Office, Ministry of Health, Rome, Italy