LONDON — AstraZeneca and Oxford University on Wednesday acknowledged a manufacturing error that has raised questions about their Covid-19 vaccine after revealing earlier this week it was “highly effective” against the disease.

On Monday, researchers said AstraZeneca vaccine had an efficacy of up to 90 percent when half a dose was administered followed by a full dose.

In a statement Wednesday, Oxford University said some of the vials used in the trial didn’t have the right concentration of vaccine so some volunteers got a half dose.

Neither Oxford University’s nor AstraZeneca’s initial public statements mentioned the dosing error, but the head of AstraZeneca's non-oncology research and development, Mene Pangalos, told Reuters on Monday that the reason for the half dose was “serendipity" and a stroke of good fortune.

“AstraZeneca/Oxford get a poor grade for transparency and rigor when it comes to the vaccine trial results they have reported,” Natalie Dean, assistant professor of biostatistics atthe University of Florida tweeted on Tuesday, adding that reporting an analysis based on a dosing error was “not desirable.”.

Experts also say a relatively small number of people in the low dose group — 2,741 people — makes it difficult to know if the effectiveness seen in the group is real or a statistical quirk, although AstraZeneca has said all of its results were statistically significant