The current SOC (state of care) generates ~3B in drug sales to SCP and (uh...uh... the other company).
This amount of sales is being driven by 3% of the US HCV population. These are the people who know they have HCV. Estimates are that only 50% of the US HCV population, is aware of their status (positive).
If VX-950 provides the 'magic bullet' for HCV treatment then the 'known' population in the US will increase.