Andrew J Pollard1,2, Else M Bijker3,4. 1. Oxford Vaccine Group, Department of Paediatrics, University of Oxford, Oxford, UK. andrew.pollard@paediatrics.ox.ac.uk. 2. NIHR Oxford Biomedical Research Centre, Oxford University Hospitals Trust, Oxford, UK. andrew.pollard@paediatrics.ox.ac.uk. 3. Oxford Vaccine Group, Department of Paediatrics, University of Oxford, Oxford, UK. 4. NIHR Oxford Biomedical Research Centre, Oxford University Hospitals Trust, Oxford, UK.