1. Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA.
2. Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305, USA.
3. VA Palo Alto Health Care System, Palo Alto, CA 94304, USA.
4. National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA.
5. Department of Surgery, Stanford University School of Medicine, Stanford, CA 94305, USA.