The Food and Drug Administration has ordered Merck & Co. to correct numerous manufacturing deficiencies at its main vaccine plant. The agency ... released a warning letter sent to Merck's chief executive, Richard T. Clark, that states FDA inspectors determined manufacturing rules are not being followed at the plant in West Point, Pa., just outside Philadelphia. The plant, which recalled two vaccines in December over sterility problems, makes a number of children's vaccines and four for adults. The nine-page letter states FDA found significant objectionable conditions" in the manufacture of vaccines and drug ingredients during repeated inspections from Nov. 26 to Jan. 17. According to the heavily redacted warning letter, Merck officials didn't thoroughly investigate when vaccine batches inexplicably failed to meet specifications, even if batches had been distributed, and some combination measles-mumps-rubella shots that failed "visual inspection for critical defects" were distributed anyway. Production of two vaccines made at West Point - PedvaxHIB, to prevent Haemophilus influenza type B, and Comvax, a combination vaccine for Haemophilus B and hepatitis B - stopped last year and 1.2 million doses of them were recalled after a sterility problem was discovered in October. The plant also makes ProQuad, which protects children against measles, mumps, rubella and chickenpox; hepatitis A, hepatitis B and meningitis vaccines for children and adults; and Gardasil, to protect young women against cervical cancer.