The Whitehouse Station, New Jersey-based company said the affected doses were distributed starting in April 2007 and that the recall is specific to these 13 lots and does not affect any other vaccines manufactured by the company.

Merck identified the potential contamination of these lots as part of the company's standard evaluation of its manufacturing processes. Sterility tests of the vaccine lots that are the subject of this recall have not found any contamination in the vaccine.

Merck noted that the potential for contamination of any individual vaccine is low. Even if present, the level of contamination would be low. These vaccines protect infants against pneumonia, meningitis and upper respiratory infections, while Comvax also prevents hepatitis B.

Individuals who received vaccine from these lots have been asked to complete their immunization series with a Haemophilus b conjugate-containing vaccine not affected by this recall. However, they do not need to be revaccinated to replace a dose they received from a recalled lot, as the efficacy of the vaccine was not affected.

Merck said it is working closely with the U.S. Food and Drug Administration and the U.S. Centers for Disease Control and Prevention to inform affected healthcare providers of this recall.

FDA said the recall was precautionary and there was no proof that the vaccine itself was contaminated.

MRK closed Wednesday's regular trade at $59.72, down $0.68 or 1.13% from the previous close, on 21.49 million shares. The stock dropped 12 cents in the post-close trade.