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Contributors TB, IL, NS and ME conceived the study. TB performed the study. IL performed the statistical analysis. TB wrote the paper. All authors approved the final draft.
Competing interests None declared.
Ethics approval Ethical approval was granted from the University of Cape Town Human Research Ethics Committee. Reference 192/2016.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Requests for data sharing should be directed to the corresponding author.