Rates of adverse events of grade 3 or 4 were similar in the two groups ( hazard ratio , 1.20 ; 95 % CI , 0.83 to 1.72 ; P = 0.33 ) , but more grade 4 adverse events occurred in the prophylaxis-stopped group ( hazard ratio , 2.04 ; 95 % CI , 0.99 to 4.22 ; P = 0.05 ) , with anemia accounting for the largest number of events ( 12 , vs. 2 with continued prophylaxis ) .