1. DSI-NRF Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA), Stellenbosch University, 19 Jonkershoek Road, Stellenbosch 7600, South Africa
2. Department of Mathematics and Applied Mathematics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
3. Department of Paediatrics and Child Health (G69), University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
4. Department of Mathematical Sciences, Stellenbosch University, Private Bag X1, Matieland 7602 and Mathematical Biosciences Group, African Institute for Mathematical Sciences, Cape Town 7945, South Africa