Funding for this research was provided by:
Medical Research Council (MC_UU_12023/24)
Received: 13 September 2018
Accepted: 5 June 2019
First Online: 25 June 2019
Ethics approval and consent to participate
: Ethical approval is not required for this study as it is a systematic review.
: Not applicable
: AS, JB, MP and CV declare no competing interests.