[1] Childrens Hosp Philadelphia, Div Infect Dis, Philadelphia, PA 19104 USA
[2] Childrens Hosp Philadelphia, Ctr Pediat Clin Effectiveness, Philadelphia, PA 19104 USA
[3] Univ Penn, Dept Pediat, Perelman Sch Med, Philadelphia, PA 19104 USA
[4] Univ Penn, Ctr Clin Epidemiol & Biostat, Perelman Sch Med, Philadelphia, PA 19104 USA