Workers at a Baltimore plant manufacturing two coronavirus vaccines accidentally conflated the vaccines’ ingredients several weeks ago, ruining about 15 million doses of Johnson & Johnson’s vaccine and forcing regulators to delay authorization of the plant’s production lines…

The mixup has halted future shipments of Johnson & Johnson doses in the United States while the Food and Drug Administration investigates. Johnson & Johnson has moved to strengthen its control over Emergent BioSolutions’ work to avoid further quality lapses.

The mistake is a major embarrassment for Johnson & Johnson, whose one-dose vaccine has been credited with speeding up the national immunization program.

It does not affect Johnson & Johnson doses that are currently being delivered and used nationwide. All those doses were produced in the Netherlands, where operations have been fully approved by federal regulators.