2.5 Data extraction
The Stata software application (version 12.0; StataCorp, College Station, TX) was used for data synthesis and analysis. Risk ratios (RR) and 95% confidence intervals (CI) were calculated; pooled RR and 95% CI were calculated using a fixed-effects model if the homogeneity assumption was not rejected (P > 0.1, I2< 50%). If not, used the random effects model. Chi-square-based Q statistic was used for subgroup analysis based on between-trial heterogeneity, and statistical significance was considered when the p-value was less than 0.05 or I2 was greater than 50%.