Merryn Voysey1, Andrew J Pollard2. 1. Oxford Vaccine Group, Department of Paediatrics, University of Oxford, Oxford OX3 7LE, UK; National Institute of Health Research, Oxford Biomedical Research Centre, Oxford, UK. 2. Oxford Vaccine Group, Department of Paediatrics, University of Oxford, Oxford OX3 7LE, UK; National Institute of Health Research, Oxford Biomedical Research Centre, Oxford, UK. Electronic address: andrew.pollard@paediatrics.ox.ac.uk.