[1] Johns Hopkins Univ, Dept Pediat, Sch Med, Baltimore, MD 21218 USA
[2] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Populat Family & Reprod Hlth, Baltimore, MD 21218 USA
[3] Univ New Mexico, Sch Med, Dept Pediat, Albuquerque, NM 87131 USA
[4] Penn State Univ, Dept Publ Hlth, Hershey, PA USA
[5] Penn State Univ, Coll Med, Dept Pediat, Hershey, PA USA