1. Department of Infectious Diseases, Instituto Nacional de Ciencias Médicas y Nutricion, 14000 Ciudad de México, DF, Mexico
2. Westat, Rockville, MD 20850-3129, USA
3. Pediatric, Adolescent and Maternal AIDS Branch, Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health, 6100 Executive Boulevard, Room 4B11, Bethesda, MD 20892-7510, USA