1. Department of Pediatrics, Program in Molecular Medicine, University of Massachusetts Medical School, Worcester,1 and
2. Statistical and Data Analysis Center, Harvard School of Public Health,2 and
3. Department of Pediatrics, New York University, New York, New York3; and
4. Boston Children’s Hospital,4 Boston, Massachusetts;
5. Division of AIDS, National Institute of Allergy and Infectious Diseases,5 and
6. Pediatric, Adolescent and Maternal AIDS Branch, National Institute of Child Health & Human Development,6 National Institutes of Health, Bethesda, Maryland