A top U.S. health official's call to split the measles-mumps-rubella (MMR) vaccine into separate shots sparked strong pushback from Merck & Co., Inc. (NYSE:MRK) and medical experts, who said such a move lacks scientific justification and could undermine immunization efforts.

Reuters reported that on Monday, acting Centers for Disease Control and Prevention (CDC) director Jim O'Neill urged vaccine makers to create individual shots for measles, mumps, and rubella.

Merck, the only manufacturer of the MMR vaccine licensed in the U.S., rejected O'Neill's proposal, stating there is no scientific evidence showing benefits from separating the shots.

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The company also warned that individual vaccines would require Food ...