Two former Merck scientists, who filed a whistleblower lawsuit accusing the drug maker of falsifying data about its mumps vaccine, say the company is refusing to answer a key question about effectiveness of the product, according to court documents.

Specifically, the scientists complain that Merck is being evasive about the actual effectiveness rate, an issue that is at the heart of their 2010 lawsuit. As noted previously, the former virologists charged the drug maker knew its vaccine was less effective than the purported...