The median overall survival was 56.5 months ( 95 % confidence interval [ CI ] , 49.3 to not reached ) in the group receiving the pertuzumab combination , as compared with 40.8 months ( 95 % CI , 35.8 to 48.3 ) in the group receiving the placebo combination ( hazard ratio favoring the pertuzumab group , 0.68 ; 95 % CI , 0.56 to 0.84 ; P < 0.001 ) , a difference of 15.7 months .