Johnson & Johnson said today (March 31) 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 U.S. 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.

According to the Times, some 15 million doses were ruined by the error.

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.