[1] St Christophers Hosp Children, Infect Dis Sect, Philadelphia, PA 19134 USA
[2] Drexel Univ, Coll Med, Philadelphia, PA 19104 USA
[3] Drexel Univ, St Christophers Hosp Children, Philadelphia, PA 19104 USA
[4] Stanford Univ, Sch Med, Palo Alto, CA 94304 USA
[5] Stanford Univ, Med Ctr, Palo Alto, CA 94304 USA