1 International Health Unit, Section of Infectious Diseases and Immunity, Imperial College London, London W12 ONS, UK. Electronic address: s.hargreaves@imperial.ac.uk.
2 International Health Unit, Section of Infectious Diseases and Immunity, Imperial College London, London W12 ONS, UK.
3 Public Health England, London, UK.
4 Department of Primary Care and Public Health, Imperial College London, London.
5 Health Protection Research Unit in Immunisation, London School of Hygiene & Tropical Medicine, London, UK.