[1] Univ Calif San Francisco, Sch Med, Dept Pediat, Div Gen Pediat, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Sch Med, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA
[3] Stanford Med Ctr, Lucile Salter Packard Childrens Hosp, Stanford, CA USA
[4] Amer Acad Pediat, Ctr Child Hlth Res, Dept Practice & Res, Pediat Res Off Settings, Elk Grove Village, IL USA
[5] Vermont Coll Med, Dept Pediat, Burlington, VT USA