1. School of Public Health, University of the Witwatersrand, Johannesburg, South Africa
2. Division of Critical Care, Charlotte Maxeke Hospital and Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa
3. National Institute for Occupational Health, National Health Laboratory Service, Johannesburg, South Africa
4. School of Pathology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa