If Merck ($MRK) has answers regarding the efficacy of its mumps vaccine, it's not being forthright, says a letter filed by an attorney representing two former Merck virologists who are now whistleblowers.

Originally filed in 2010, the suit claims that the pharma giant skewed the tests of its U.S.-exclusive mumps vaccine by adding animal antibodies to blood samples, resulting in 95% efficacy that kept competitors--who couldn't match the efficacy--off the market. And since the original suit, the new filing claims, Merck has been "consistently evasive," in providing answers, saying it can't run a new clinical trial to determine the efficacy and only providing 50-year-old data on the jab.

Instead, Merck has been "hiding behind a facade of confusion and obfuscation as to what efficacy means," the letter states, even though the attorneys say "efficacy is a common term used throughout the industry to identify how well a vaccine works. It is also a term Merck has, until now, freely used throughout these proceedings to describe how well its vaccine works."

Merck, for its part, says the lawsuit is "completely without merit," adding that "nothing is more important to Merck than the safety and effectiveness of our vaccines and medicines and the people who use them."

"Merck's mumps vaccine led to a 98 percent reduction in the incidence of mumps in the United States, according to the US CDC," a spokesperson wrote to FierceVaccines. "Our vaccine is the only vaccine for mumps ever used in the United States. The labeling for M-M-R®II accurately reflects the safety and efficacy of the vaccine as approved by the U.S. FDA."

The whistleblowers' attorneys asked Judge Lynne Sitarski of the U.S. District Court in the Eastern District of Pennsylvania to compel Merck to provide answers on the vaccine's current efficacy and personnel involved in the pharma's response.

The exclusive jab in question, MMRII, also protects against measles and rubella, bringing Merck $721 million last year.

Merck R&D chief Roger Perlmutter

If the allegations aren't enough, the pharma giant has been battling antivaccination fears of late, with concerns mounting nationwide over a perceived connection between vaccines to autism, charged debates over religious exemptions and a related measles outbreak in California. In a response, R&D chief Roger Perlmutter said in February Merck can only do so much to encourage vaccinations because it's a "commercial entity."

Perlmutter argued that the government and other advocacy groups would be better suited to encourage vaccinations and ease antivaccination fears.

- here's the letter and original suit

- more from Pharmalot and a story from Reuters

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