1. Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
2. Walter Reed Army Institute of Research, Silver Spring, MD 20910, USA.
3. Bioqual Inc., Rockville, MD 20852, USA.
4. Ragon Institute of Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard, Cambridge, MA 02139, USA.