1. Center for Child Health Care Studies, Department of Ambulatory Care and Prevention, Harvard Pilgrim Health Care and Harvard Medical School, Boston, Massachusetts
2. Division of Infectious Diseases, Children's Hospital Boston, Boston, Massachusetts
3. Department of Society, Human Development, and Health, Harvard School of Public Health, Boston, Massachusetts
4. Department of Biostatistics, Harvard School of Public Health, Boston, Massachusetts
5. Department of Pediatrics, University of Medicine and Dentistry of New Jersey, Newark, New Jersey