1. Division of Infectious Diseases, Department of Internal Medicine, University of Nebraska Medical Center, Omaha, Nebraska.
2. Statistical Data Analysis Center, Harvard School of Public Health, Boston, Massachusetts.
3. Division of AIDS, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland.
4. Division of Infectious Diseases, Department of Pediatrics and Department of Medicine, University Hospitals Cleveland Medical Center and Case Western Reserve University, Cleveland, Ohio.