1. Medical Research Council, Respiratory and Meningeal Pathogens Research Unit, Johannesburg, South Africa
2. Department of Science and Technology/National Research Foundation, Vaccine Preventable Diseases Research, Faculty of Health Sciences, University of Witwatersrand, Johannesburg, South Africa
3. Centre for Vaccines and Immunology, National Institute for Communicable Diseases of the National Health Laboratory Service, Johannesburg, South Africa
4. University of Colorado School of Medicine, Colorado School of Public Health, Aurora