[1] Karolinska Inst, Dept Med Epidemiol & Biostat, S-17177 Stockholm, Sweden
[2] Orebro Univ Hosp, Dept Paediat, Orebro, Sweden
[3] Univ Nottingham, Sch Med, Div Epidemiol & Publ Hlth, Nottingham, England
[4] Columbia Univ Coll Phys & Surg, Dept Med, New York, NY USA