Ten studies were included.
The figures given below relate to the number of studies, the pooled failure rate of those cases exposed (%), and the pooled 95% confidence intervals (CIs) for secondary failure rate.
Group A: 1 study; failure rate 3 out of 13 (23.1%), 95% CI: 5.05, 53.81
Group B: 3 studies; 32 out of 572 (5.59%), 95% CI: 3.86, 7.81
Group C: 6 studies; 0 out of 2,061 (0.0%), 95% CI: 0, 0.15
The grouping of studies appeared robust as the transfer of one study to another group resulted in significant heterogeneity within that group. The reasons for heterogeneity between the studies is analysed by a narrative discussion in the authors' text.