MRC Centre for Outbreak Analysis and Modelling, DIDE, Imperial College, London, United Kingdom.|Oxford University Clinical Research Unit-Wellcome Trust Major Overseas Programme, Ho Chi Minh City, Vietnam.|Oxford University Clinical Research Unit-Wellcome Trust Major Overseas Programme, Ho Chi Minh City, Vietnam.|MRC Centre for Outbreak Analysis and Modelling, DIDE, Imperial College, London, United Kingdom.|Oxford University Clinical Research Unit-Wellcome Trust Major Overseas Programme, Ho Chi Minh City, Vietnam.|MRC Centre for Outbreak Analysis and Modelling, DIDE, Imperial College, London, United Kingdom.