Fig. 2

a Risk of bias graph exhibiting the review of the authors’ judgments about each risk of bias item presented as percentages across all included studies. b Risk of bias summary revealing the review of the authors’ judgments about each risk of bias item for included RCTs. Minus sign represents the risk of bias present, plus sign indicates the risk of bias absent, and question mark equals the risk of bias uncertain. c The funnel plots of the included studies. RR, relative risks; SE, standard error