Merck & Co’s week of woe has continued with the news that US regulators have told the firm to correct what are described as “numerous manufacturing deficiencies” at its main vaccines plant.

The US Food and Drug Administration has issued a warning letter to Merck having conducted an inspection of the firm’s West Point, Pennsylvania facility between November 26, 2007 and January 17 this year. The agency’s investigators say they documented “significant deviations from current good manufacturing practice” at the plant in the manufacture of vaccine products, bulk drug substances and drug components.

The products manufactured at the West Point site include Merck & Co’s cervical cancer vaccine Gardasil and ProQuad, its combination jab against measles, mumps, rubella and chicken pox. Also produced at the facility are the Haemophilus influenzae type B vaccine PedvaxHIB and ComVax for haemophilius b and hepatitis B and 1.2 million doses of these two jabs were voluntarily recalled in December because Merck said it could not assure sterility of those specific vaccine lots.

The warning letter, which is posted on the FDA’s website in a heavily edited form, has been issued because Merck's response to the FDA’s initial inspection report was "inadequate to address the serious deviations noted”. Most of the myriad complaints appear to deal with procedural errors but the agency stresses it "does not believe that the issues identified will affect the safety of the vaccines" made at the facility.

Merck says that it is committed to working with the FDA to address these issues, adding that it does not distribute contaminated products and is confident in their “quality, effectiveness and safety”.

The letter comes in a week when Merck’s stock has taken a bashing after the FDA rejected applications firstly for a proposed fixed combination of Singulair (montelukast) and S-P’s Claritin (loratadine) as a treatment for allergic rhinitis and secondly for its new cholesterol compound MK-0524A (extended-release niacin/laropiprant).