1. San Diego VA Healthcare System and University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093–0679, USA.
2. Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
3. Project Inform, 1375 Mission Street, San Francisco, CA 94103, USA.
4. Gladstone Institute of Virology and Immunology, San Francisco, CA 94158, and University of California at San Francisco, San Francisco, CA 94143, USA.
5. Merck and Co., West Point, PA 19486, USA.