This effect was conditioned on interactions with severity ( t451 = 1.97 ; P = .05 ; NNT ,5 ) and chronicity ( 2 = 7.46 ; P = .02 ; NNT ,6 ) such that the advantage for combined treatment was limited to patients with severe , nonchronic MDD ( 81.3 % vs 51.7 % ; n = 146 ; t145 = 3.96 ; P = .001 ; HR , 2.34 ; 95 % CI , 1.54-3 .57 ; NNT , 3 ; 95 % CI , 2-5 ) .