1. Division of Viral Products, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD 20892, USA.
2. Laboratory of Viral Diseases, National Institutes of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
3. Novartis Vaccines and Diagnostics GmbH, Marburg 35041, Germany.
4. Novartis Vaccines and Diagnostics Research Center, Siena 53100, Italy.