Via the New York Times, this crummy news (emphasis mine):
WASHINGTON — Workers at a plant in Baltimore manufacturing two coronavirus vaccines accidentally conflated the ingredients several weeks ago, contaminating up to 15 million doses of Johnson & Johnson’s vaccine and forcing regulators to delay authorization of the plant’s production lines. The plant is run by Emergent BioSolutions, a manufacturing partner to both Johnson & Johnson and AstraZeneca, the British-Swedish company whose vaccine has yet to be authorized for use in the United States. Federal officials attributed the mistake to human error...
...The problems arose in a new plant that the federal government enlisted last year to produce vaccines from Johnson & Johnson and AstraZeneca. The two vaccines use the same technology employing a harmless version of a virus — known as a vector — that is transmitted into cells to make a protein that then stimulates the immune system to produce antibodies. But Johnson and Johnson’s and AstraZeneca’s vectors are biologically different and not interchangeable.
In late February, one or more workers somehow confused the two during the production process, raising questions about training and supervision. In the past year, Emergent has hired and trained hundreds of new workers to produce millions of doses of both vaccines that were supposed to be ready by the time clinical trials showed whether the vaccines actually worked.
Vaccine production is a notoriously fickle science, and errors are often expected to occur and ruin batches. But Emergent’s mistake went undiscovered for days until Johnson & Johnson’s quality control checks uncovered it, according to people familiar with the situation. By then, up to 15 million doses had been contaminated, the people said.
None of the doses ever left the plant, and the lot has been quarantined.
I'm not surprised this happened - I can't imagine the pressure that Emergent is under (especially if they're manufacturing two of the COVID vaccines). As I said on Twitter, I expect that the root cause will be found far from the plant floor.* Here's hoping the relevant folks there dust themselves off and get back to their important work. Best wishes to them, and to all of us.
*Bad day to be on the QA investigation team there for sure.