Pharmasset ( VRUS) surged on Monday after the company announced positive data from a short-term early stage study on its hepatitis C candidate, R7128.Shares of the Princeton, N.J.-based clinical-stage company rose $3.36, or 24%, to $17.35, on the data. The four-week trial assessed two doses of Pharmasset's R7128 in combination with the standard of care, Pegasys plus Copegus, in 50 chronically infected patients who hadn't received prior treatment. Pharmasset said 85% of patients given the higher dose, 1,500 milligams, and 30% at the lower 500 milligram dose had undetectable levels of the virus after four weeks. For comparison, 10% of patients on the placebo, which was standard of care alone, achieved undetectable levels of the virus. Safety and tolerability of the Pharmasset drug combination were comparable to the placebo, according to the company. In September, Pharmasset and Roche provided data on a phase I, two-week study on R7128 as a monotherapy for patients who failed to respond to standard therapy. The drug showed a 99% decrease in hepatitis C with no serious adverse events -- the companies said they were hoping the results would translate when the drug was used in combination with other therapies in longer studies and untreated populations. Pharmasset will provide a company-overview at the JPMorgan Healthcare Conference in San Francisco on Thursday.