Merck's bulk drug substances used to manufacture many of its vaccines are adulterated, and the FDA is requesting a meeting with senior management to expedite the firm's corrective actions, according to a recent warning letter.

The FDA cited the company for numerous GMP violations at its vaccine production operations, including those for human papillomavirus vaccine Gardasil and combination measles, mumps, rubella and chicken pox vaccine ProQuad.

The warning letter resulted from an inspection at the firm's West Point, Penn., facility between Nov. 27, 2007, and Jan. 17. The facility produces most of Merck's vaccines, the company said. The inspection resulted in 49 Form 483 observations.

In a question-and-answer statement, the FDA said the deviations do not affect the safety of Merck vaccines, and no shortages are expected.

Merck has been plagued with manufacturing problems for its vaccines containing varicella. Potency issues for bulk varicella have created supply disruptions for Zostavax and ProQuad, which is not available for purchase.

During Merck's first quarter earnings call, CEO Richard Clark said the FDA is reviewing regulatory submissions for a new varicella manufacturing process.

The warning letter can be accessed here.