1. Departments of Pediatrics and Medicine, University of Alabama at Birmingham, Birmingham, Alabama 35294
2. Westat, Rockville, Maryland 20850
3. Department of Pediatrics, University of Pennsylvania, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104
4. Department of Pediatrics, University of California at San Francisco, San Francisco, California 94118
5. Center for Virology, Immunology and Infectious Disease Research, Children's National Medical Center, Washington, D.C. 20010