1. Department of Infectious Disease, MedImmune, Gaithersburg, MD 20878, USA.
2. Department of Translational Science, MedImmune, Gaithersburg, MD 20878, USA.
3. Department of Medicine, University of California, San Francisco, San Francisco, CA 94110, USA.
4. Division of Infectious Diseases and Vaccinology, School of Public Health, University of California, Berkeley, Berkeley, CA 94720, USA.