[1] Stockholm South Gen Hosp, Sachs Children & Youth Hosp, Stockholm, Sweden
[2] Karolinska Inst, Dept Clin Sci & Educ Sodersjukhuset, Stockholm, Sweden
[3] Karolinska Inst, Dept Med Solna, Clin Epidemiol Unit, Stockholm, Sweden
[4] Karolinska Inst, Inst Environm Med, Unit Biostat, Stockholm, Sweden
[5] Karolinska Inst, Dept Med Epidemiol & Biostat, Solna, Sweden
[6] Orebro Univ Hosp, Dept Pediat, Orebro, Sweden
[7] Univ Nottingham, Sch Med, Div Epidemiol & Publ Hlth, Nottingham, England
[8] Columbia Univ Coll Phys & Surg, Dept Med, New York, NY USA