UPDATE 1-Pharmasset hep C drug shows efficacy in mid-stage trial

* Study compared drug candidate PSI-7977 with dummy drug

* Patients receiving drug showed rapid virologic response

May 4 (Reuters) - Pharmasset Inc VRUS.O said a mid-stage study of its experimental hepatitis C treatment showed the drug had short-term antiviral activity and was safe and well tolerated.

The company said it plans to quickly progress the drug -- PSI-7977 -- into a second mid-stage study in the fourth quarter, to generate longer-term efficacy and safety data.

The mid-stage study compared three doses of the drug with a dummy drug. Both PSI-7977 and the dummy drug were given in combination with standard-of-care.

Following 28 days of treatment, 88 percent of patients receiving a 100 mg dose had virus levels below the limit of detection, while 94 percent of patients receiving a 200 mg dose achieved that level.

Of patients receiving a 400 mg dose, 93 percent had virus levels below the detection limit, while only 21 percent of patients who were administered the dummy drug, saw virus levels fall below the detection limit.

There were no serious adverse events reported during the 28-day treatment period, and no adverse events lead to treatment discontinuation.

All adverse events reported were of mild to moderate intensity, of which a majority were mild, the company said in a statement.

Shares of the company closed at $32.74 Monday on Nasdaq. (Reporting by Esha Dey in Bangalore; Editing by Roshni Menon)