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. That efficacy rate is on par with other vaccine candidates announced by Pfizer and Moderna earlier this month. In the group that got two full doses, the vaccine appeared to be 62 percent effective. Combined, the drugmakers said the vaccine appeared to be 70 percent effective.
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. The university said that it discussed the problem with regulators, and agreed to complete the late stage trial with two groups. The manufacturing problem has been corrected, according to the statement.
After Bloomberg reported on Thursday that the CEO of AstraZeneca said there would likely be an additional trial after the recent results, an AztraZeneca spokesperson clarified in an email that the late stage trial is "not a brand new trial."
"As we communicated in Monday's press event there is strong merit in continuing to further investigate the half dose/full dose regimen," the company said.
The news of the dosing error comes as the world is focused on Covid-19 vaccines to help end the pandemic that has claimed 1.4 million lives.
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 told NBC News in an emailed statement on Thursday that the studies were conducted “to the highest standards.”
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But the way in which the results were arrived at and reported has led to questions from experts.
“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.”
Another point of confusion comes from a decision to pool results from two groups of participants who received different dosing levels to reach an average 70 percent effectiveness, David Salisbury, and associate fellow of the global health program at the Chatham House think tank, told the Associated Press.
“You’ve taken two studies for which different doses were used and come up with a composite that doesn’t represent either of the doses,″ he said of the figure. “I think many people are having trouble with that.″
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.
However, some experts point out that the vaccine trials are an experiment, and serendipitous findings are normal.
“This was a very fortunate error,” said Nicola Stonehouse, professor of molecular virology at the University of Leeds. “It’s an experiment, and sometimes experiments don’t go the way you think. Sometimes things are unexpected. But they realized what happened, and that’s good experimental science.”
But Stonehouse cautioned that so far only a summary of the results, not the actual data, have been released, and rigorous scrutiny by regulators will need to follow to ensure the results don't undermine public confidence in the global effort to vaccinate billions of people.
The Associated Press and Reuters contributed to this report.