The authors acknowledge the support of the Nigeria Federal Ministry of Health and the WHO Nigeria Country Office in the conduct of this study. Special thanks to Victoria Nyawira Nyaga for checking through our statistical analyses. Authorship declaration: DA conceived and designed the study. DA, DQA and AA conducted the literature searches and data extraction. DA, DQA and EOO wrote the first draft. DA and MBA conducted the analysis. DA, AA, RGM, MTD, CO, NE, MOH, WA and IFA contributed to the final draft and checked for important intellectual content. All authors approved the manuscript as submitted.