[1] Univ London Imperial Coll Sci Technol & Med, Sch Publ Hlth, Dept Infect Dis Epidemiol, London W2 1PG, England
[2] CPO Piemonte, SCDU Epidemiol Tumori, Serv Stat Med, I-28100 Novara, Italy
[3] CPO Piemonte, Epidemiol Tumori, I-10100 Turin, Italy
[4] Univ London Imperial Coll Sci Technol & Med, Dept Epidemiol & Publ Hlth, London W2 1PG, England