To pad out the reasoning behind the investigator's analysis,...

To pad out the reasoning behind the investigator's analysis, with sofosbuvir Gilead have purchased a polymerase inhibitor that is highly specific for the viral enzyme and has no significant activity on mitochondrial enzymes which tends to underlie toxicity of antivirals and antibiotics for that matter. There has never been a case of resistance to sofosbuvir as mutation at its site of activity appears to always render the virus impotent. It's potency is such that on treatment ALL patients will go virus negative after a length of treatment - the difference in the amount of time that takes depends on genotype and degree of liver injury. This also occurs with PEG RIBA and is why there are different treatment durations. It is actually likely that sofosbuvir mono therapy if given for long enough would cure all patients but that is untidy as trials and doctors like specific durations of treatment. The use of ledipasvir allows shortening of treatment to a few months. It too has minimal toxicity and no significant interactions. The reason you don't yet see cross the board 100% cure rates is that the optimal duration of therapy has not been determined for each scenario. Gilead currently have a wide range of treatment arms to determine that data. The question does now appear to be not if but when.