1. Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA.
2. Harvard Medical School, Boston, MA 02115, USA.
3. Department of Obstetrics and Gynecology, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.
4. Massachusetts Department of Public Health, Boston, MA 02108, USA.
5. Ragon Institute of MGH, MIT, and Harvard, Cambridge, MA 02139, USA.