At a median follow-up of 9.4 months , ibrutinib significantly improved progression-free survival ; the median duration was not reached in the ibrutinib group ( with a rate of progression-free survival of 88 % at 6 months ) , as compared with a median of 8.1 months in the ofatumumab group ( hazard ratio for progression or death in the ibrutinib group , 0.22 ; P < 0.001 ) .