[1] Orebro Univ Hosp, Dept Pediat, Orebro, Sweden
[2] Karolinska Univ Hosp, Karolinska Inst, Clin Epidemiol Unit, Stockholm, Sweden
[3] Orebro Univ Hosp, Clin Res Ctr, Orebro, Sweden
[4] Univ London Imperial Coll Sci Technol & Med, Charing Cross Hosp, Dept Primary Care & Social Med, London, England