p16-positive tumor status ( HR , 0.48 ; 95 % CI , 0.31 to 0.74 ) and receipt of salvage surgery ( HR , 0.48 ; 95 % CI ; 0.27 to 0.84 ) reduced risk of death after disease progression whereas distant versus locoregional progression ( HR , 1.99 ; 95 % CI , 1.28 to 3.09 ) increased risk , after adjustment for tumor stage and cigarette pack-years at enrollment .