The stock lost $4.42, or 15.3 percent, to $24.48 in morning trading. Earlier, the shares traded as low as $22.92, after ranging from $25.61 to $45.38 over the past year.

The stock retreated 8.7 percent Friday after Vertex reported mixed results from a mid-stage clinical trial of the drug, and competitor Roche said its drug Pegasys lowered virus levels for patients who did not respond strongly to other drugs.

Leerink Swann & Co. analyst Howard Liang downgraded Vertex to "Market Perform" from "Outperform," and lowered his price target to $30 per share from a range of $43 to $45.

"We believe the competitive news could further affect the sentiment on the stock," he said.

Liang expects positive results for a potential competing hepatitis drug candidate being developed by Johnson & Johnson and Medivir. He said there are three similar drugs in early clinical testing.

Rodman & Renshaw analysts Navdeep Jaikaria and Michael King downgraded Vertex stock to "Market Perform" from "Market Outperform."

"We do not see many near term positive drivers for Vertex while competitive risks remain," they said. "Further, we are concerned that the Food and Drug Administration may take a more stringent stand and ask Vertex to complete 48 weeks of standard of care treatment for the control arm before telaprevir new drug application filing."