The percent of subjects who maintained virologic suppression in the TDF versus d4T or ZDV group at Week 24 were 93.8 % versus 89.8 % ( difference 4.0 % ; 95 % confidence interval : : -6.9 % to 14.9 % ) and at Week 48 were 83.3 % versus 91.8 % ( difference : -8.5 % ; 95 % confidence interval : -21.5 % to 4.5 % ; missing = failure , intent-to-treat analysis ) .