Funding for this research was provided by:
Wellcome Trust (107754/Z/15/Z)
Received: 25 April 2018
Accepted: 22 May 2019
First Online: 4 June 2019
Ethics approval and consent to participate
: Ethical approval under clearance certificate number M1611104 was received from the Human Research Ethics Committee of the University of the Witwatersrand. Consent to participate not applicable.
: Not applicable.
: The authors declare that they have no competing interests.