Vertex Pharmaceuticals (Nasdaq: VRTX) investors have a new reason to celebrate. Yesterday, at the European Association for the Study of the Liver (EASL) conference, Vertex reported heartening new clinical trial data for its lead drug, telaprevir.

Vertex has conducted two main phase 2 studies on this potential treatment for newly diagnosed chronic hepatitis C (HCV) patients. The initial PROVE-1 study produced positive data. However, interim efficacy results from the subsequent PROVE-2 study, first announced last November, seemed less thrilling at first.

After 48 weeks of treatment, 65% of patients receiving telaprevir in PROVE-2 showed no signs of the HCV in their bloodstream, indicating that they'd been cured. But 59% of patients in the control group showed similarly promising signs -- on week 36 of their 48 weeks of standard HCV treatment. Investors didn't like this news at the time, especially given PROVE-1's far better results. Wall Street promptly knocked Vertex's shares down 17% after the data came out.

Yesterday, Vertex dispelled any ambiguity about the PROVE-2 results. Its new efficacy data showed that 68% of telaprevir-treated patients were ultimately cured of hepatitis C, compared to only 48% of patients in the control group. Obviously, that's a far more promising outcome, and it matches PROVE-1's 20-percentage-point improvement on current treatments' cure rates.

The new PROVE-2 is exciting, as is the Vertex 107 study data the company presented at EASL. If Vertex shares have remained in a slump recently, it may owe to Schering-Plough (NYSE: SGP). The rival pharmaceutical previously amassed very positive data on its own hepatitis C protease inhibitor drug, boceprevir, and it's been hinting at more to come at EASL. According to the abstract of a previous phase 2 study (search for "SPRINT-1" at this link), boceprevir reduced hepatitis C viral loads to undetectable levels in 79% and 69% of patients in two large 200-person-plus patient groups after 12 weeks of treatment.

Both boceprevir and telaprevir are generally producing strong efficacy data in most of their clinical studies to date, but Vertex may still have the lead in the race to become the first hepatitis C antiviral therapy on the market.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A-plus disclosure policy.