WASHINGTON (BLOOMBERG) - A manufacturing error at a plant involved in Covid-19 vaccine production affected 15 million doses worth of an ingredient for Johnson & Johnson's vaccine, according to two sources familiar with the matter, but the company downplayed the situation and said it met its most recent vaccine delivery target.

The issue, which occurred recently at a Baltimore Emergent BioSolutions facility, was first reported by the New York Times on Wednesday.

It is not expected to change President Joe Biden's expectation that the United States will have enough vaccine for all adults in May.

In a statement, Johnson & Johnson said a batch of drug substance failed its quality test.

"This quality control process identified one batch of drug substance that did not meet quality standards at Emergent BioSolutions, a site not yet authorised to manufacture drug substance for our Covid-19 vaccine. This batch was never advanced to the filling and finishing stages of our manufacturing process," Johnson & Johnson said.

The company said it addressed the issue with Emergent BioSolutions and reported it to the Food and Drug Administration.

Johnson & Johnson said it beat its March delivery target, providing the US government more than 20 million doses, and that it expects to deliver another 24 million by the end of April.

Mr Biden has set a goal for almost 100 million doses by the end of May.

Johnson & Johnson said it still plans to deliver 100 million by the end of June, "aiming to deliver those doses by the end of May".

Johnson & Johnson is now expanding its presence in the Emergent BioSolutions facility, according to the statement.

"Johnson & Johnson is providing additional experts in manufacturing, technical operations and quality to be on-site at Emergent to supervise, direct and support all manufacturing of the Johnson & Johnson Covid-19 vaccine," it said.

Representatives for Emergent BioSolutions declined to comment, deferring to Johnson & Johnson's statement.

Johnson & Johnson has committed to delivering 20 million doses by the end of March, and 100 million by the end of June, though Mr Biden had said that he expected nearly all of those to arrive by the end of May. He relied on that in moving up his timeline on when the US would have enough vaccine to cover all adults.

The White House said last week it expected a surge of 11 million doses this week from Johnson & Johnson. It is not clear if those are among the affected doses.

The two other authorised vaccine producers, Pfizer-BioNTech and Moderna, are on schedule for their deliveries.