BOSTON — In the 10 weeks since the U.S. Food and Drug Administration granted approval to a new hepatitis C drug from Vertex Pharmaceuticals Inc., the rollout of it is beating analysts’ expectations.

The drug is being sold and marketed under the name Incivek. The launch is considered a key test for Vertex, because it’s the Cambridge biotech company’s first major commercial rollout.

“It looks like they’ve had an excellent start,” said Howard Liang, an analyst at the Boston investment bank Leerink Swann. He’s been watching how well Incivek is performing against a competing drug from Merck that is rolling out at the same time.

“It looks like the split is 80/20 in favor of Vertex,” Liang said of the early prescription sales.

Besides the proportion, the number of sales is higher than Liang expected, too.

Biotech drug launches are more difficult than many people realize and often disappoint.

However, Vertex appears to be off to a strong start.

“While it is still the early days,” Liang wrote in a report to investors, “the current script trends coupled with the initial actual reported sales seem to suggest that Incivek launch has the potential to well exceed expectations and position it as one of the highly successful biotech product launches.”

By Curt Nickisch