We also noted significant cross-protective vaccine efficacy against 6-month persistent infection with HPV 31 ( 791 % , 977 % CI 276-959 ) and HPV 45 ( 769 % , 185-956 ] ) Serious adverse events occurred in 285 ( 10 % ) of 2881 women in the vaccine group and 267 ( 9 % ) of 2871 in the control group ; five ( < 1 % ) and eight ( < 1 % ) of these events , respectively , were believed to be related to vaccination .