1. Instituto Gulbenkian de Ciência, Rua da Quinta Grande 6, 2780-156 Oeiras, Portugal
2. Division of Virology, Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, UK
3. Division of Virology, National Institute for Biological Reagents and Control, Medicines and Healthcare Products Regulatory Agency, Hertfordshire, UK