1. Vaccine Testing Center, Department of Medicine, University of Vermont College of Medicine, Burlington, VT 05401, USA.
2. National Institutes of Allergy and Infectious Diseases, Bethesda, MD 20892, USA.
3. Center for Immunization Research, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205, USA.
4. Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.