1. CEESCAT, Hospital Universitari Germans Trias i Pujol, Badalona, Spain
2. Division of Infectious Diseases, C.H.U. Saint Pierre, Brussels, Belgium
3. Observatoire Régional de la Santé, ORS PACA, Marseilles, France
4. Dept.of Epidemiology of the National Institut of Infect.Diseases. INMI "L. Spallanzani" IRCSS, Rome, Italy
5. Public Health Laboratory Service Communicable Disease Surveillance Centre, London, United Kingdom