Paul Klenerman1, Narayan Ramamurthy2. 1. NIHR Biomedical Research Centre and Nuffield Department of Medicine, Peter Medawar Building for Pathogen Research, University of Oxford, Oxford, UK. Electronic address: paul.klenerman@ndm.ox.ac.uk. 2. NIHR Biomedical Research Centre and Nuffield Department of Medicine, Peter Medawar Building for Pathogen Research, University of Oxford, Oxford, UK.