Johnson & Johnson said on Wednesday it had found a problem with a batch of drug substance for its Covid-19 vaccine, which did not meet quality standards at Emergent Biosolutions’ production site in Baltimore, Maryland.

The issue was identified and addressed with Emergent and shared with the US Food & Drug Administration, J&J said, adding it was sending more people to supervise manufacturing at the plant.

Workers at the plant manufacturing coronavirus shots for J&J and AstraZeneca accidentally conflated the vaccines’ ingredients several weeks ago, the New York Times earlier reported, adding that Federal officials attributed the mistake to human error.

Johnson & Johnson said in a statement to CNN Wednesday that the quality control process at the plant identified “one batch” of drug substance that did not meet quality standards. The batch in question was part of a test run and quality check. The site is not yet authorized by the FDA to make the drug substance used in the vaccine.

J&J tapped contract manufacturers Catalent Inc and Emergent to scale up production and meet its global supply targets. Catalent provides the final stage – called fill and finish – while Emergent makes the drug substance.

The company also said it had met its commitment to deliver more than 20 million single-shot vaccines by the end of March in the United States.

The mistake will not affect the doses that are being delivered and used nationwide, as those doses were produced in the Netherlands. However, all future shipments of the vaccine — some tens of millions of doses in the next month — were set to come from the Baltimore plant.

Nonetheless, federal officials expect to have enough doses to meet President Biden’s goal of securing enough vaccine to immunize every American adult by the end of May as Pfizer-BioNTech and Moderna are delivering vaccines on or ahead of schedule.