In the per-protocol cohort , the QIV efficacy was 55.4 % ( 95 % CI , 39.1 to 67.3 ) , and the efficacy against culture-confirmed influenza 55.9 % ( 97.5 % CI , 35.4 to 69.9 ) ; the efficacy among children with moderate-to-severe influenza was 73.1 % ( 97.5 % CI , 47.1 to 86.3 ) .