Methods
The protocol of this review was registered in PROSPERO (CRD42018080575) on 1 December 2017. We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement and the Meta-analysis of Observational Studies in Epidemiology (MOOSE) group. None of the authors received specific funding for this review. There was no patient or public involvement in the development or implementation of this review.