1. Nuffield Department of Clinical Medicine, Peter Medawar Building for Pathogen Research, University of Oxford, Oxford OX1 3SY, United Kingdom
2. Department of Virology, Max von Pettenkofer Institute, 80336 Munich
3. Department of Virology, Robert Koch-Institute, 13353 Berlin, Germany
4. Department of Pathology, Institute for Experimental Immunology, 8091 Zürich, Switzerland