[1] Harvard Univ, Sch Publ Hlth, Ctr Biostat AIDS Res, Boston, MA 02115 USA
[2] Harvard Univ, Sch Publ Hlth, Dept Biostat, Boston, MA 02115 USA
[3] Harvard Univ, Sch Publ Hlth, Dept Epidemiol, Boston, MA 02115 USA
[4] Univ Washington, Sch Med, Dept Pediat, Seattle, WA 98195 USA
[5] Eunice Kennedy Shriver Natl Inst Child Hlth & Hum, Bethesda, MD USA
[6] Univ Maryland, Sch Med, Dept Pediat, Baltimore, MD 21201 USA