[1] Int Epidemiol Assoc, Natl Res Council, I-56100 Pisa, Italy
[2] Inst Municipal Invest Med, E-08003 Barcelona, Spain
[3] Danish Epidemiol Sci Ctr, DK-8000 Aarhus, Denmark
[4] Univ London London Sch Hyg & Trop Med, London WC1E 7HT, England
[5] Harvard Univ, Boston, MA 02115 USA
[6] Univ Athens, Boston, MA 02115 USA