Johnson & Johnson said Wednesday a quality control issue at a vaccine manufacturing plant in Maryland has delayed the delivery of millions of vaccine doses.

The vaccine manufacturer issued a statement explaining the quality control process identified one batch of drug substance at Emergent Biosolutions in Maryland “did not meet quality standards” and the doses were therefore never advanced to the filing and finishing stages.

“This is an example of the rigorous quality control applied to each batch of drug substance. The issue was identified and addressed with emergent and shared with the United States Food & Drug Administration,” Johnson & Johnson said.

Workers at the plant, which is manufacturing vaccines for both Johnson & Johnson and AstraZeneca, mixed up ingredients in the two vaccines several weeks ago, ruining 15 million doses of Johnson & Johnson’s vaccine, a US-based daily reported.

Johnson & Johnson said that Emergent had not yet received Emergency Use Authorization to manufacture the drug substance following the incident.

Emergent representative James Glover told a television network that no vaccine doses have been shipped out of the facility.

“Until we receive EUA (emergency use approval), those products won’t be distributed,” said Glover. “It hasn’t impacted us as a facility. We continue to manufacture at the same cadence we’ve manufactured for months now.”

Johnson & Johnson added that it would provide additional experts in manufacturing, technical operations and quality to be on-site at Emergent to supervise, direct and support further manufacturing of its vaccine.