Ethical approval
The study was approved by the LSHTM Ethics Committee (approval number 6296).
Funding
None declared.
Competing interests
The authors have no relevant interests to declare.
Contributors
SM, MP, JG each contributed to the design of the study; SM and JG were involved in the data collection and analysis; all authors were involved in drafting, revising and approving the
final paper. SM will act as guarantor.