1. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.
2. Departments of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205.
3. Departments of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205.
4. Department of Nutritional Science, University of California, Berkeley, California 94720 and University of California, San Francisco, California 94143.
5. Departments of Epidemiology and Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205.