"Merck should not be permitted to raise as one of its principal defenses that its vaccine has a high efficacy, which is accurately represented on the product's label, but then refuse to answer what it claims that efficacy actually is," the letter said.

A representative of Merck could not immediately be reached for comment.

The two scientists, Stephen Krahling and Joan Wlochowski, filed their whistleblower lawsuit in 2010 claiming Merck, the only company licensed by the Food and Drug Administration to sell a mumps vaccine in the United States, skewed tests of the vaccine by adding animal antibodies to blood samples.

As a result, they said, Merck was able to produce test results showing that the vaccine was 95 percent effective, even though more accurate tests would have shown a lower success rate. The plaintiffs said these false results kept competitors from trying to produce their own mumps vaccines, since they were unable to match the effectiveness Merck claimed.

In 2012, Alabama-based Chatom Primary Care and two individual doctors, all purchasers of the vaccine, filed a proposed antitrust class action based on the allegations in the whistleblower suit. The two suits are now being coordinated before U.S. District Judge C. Darnell Jones and Magistrate Judge Sitarski.

The case is United States ex rel Krahling et al v. Merck & Co Inc, U.S. District Court, Eastern District of Pennsylvania, No. 10-4374.

Source: http://www.reuters.com/article/2015/06/04/health-vaccine-idUSL1N0YQ0W820150604