Idenix Pharmaceuticals Inc. soared Wednesday, after the company reported positive midstage trial data of its hepatitis C drug, suggesting it was closer to competing with larger drug makers. The large market for hepatitis C, which is expected to be worth $15 billion by 2019, has generated significant investor interest. Data from a midstage trial showed that nearly all patients who responded to Idenix’s experimental treatment were able to sustain their defense against the virus.