AstraZeneca said it would publish more data on its American clinical trial “within 48 hours” after the independent monitoring board that oversaw the study warned US authorities that results released by the company on Monday were misleading.

In a rare move, the US National Institute of Allergy and Infectious Diseases said it had been notified by the trial’s data and safety monitoring board (DSMB) that “it was concerned by information released by AstraZeneca on initial data from its Covid-19 vaccine clinical trial”.

“The DSMB expressed concern that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data,” the NIAID said on Tuesday morning.

In response, AstraZeneca said the published results were based on a “pre-specified interim analysis” of data up to February 17.

“We have reviewed the preliminary assessment of the primary analysis and the results were consistent with the interim analysis,” AstraZeneca said. “We will immediately engage with the independent data safety monitoring board (DSMB) to share our primary analysis with the most up to date efficacy data.”

“We intend to issue results of the primary analysis within 48 hours,” it added.

The DSMB is an independent board of experts appointed to oversee ongoing clinical trials. In this case the DSMB had worked with AstraZeneca on its long-awaited American trial, the results of which were released on Monday.

The trial showed the vaccine was 79 per cent effective at stopping symptomatic Covid-19 and 100 per cent effective at preventing people from falling seriously ill. It also identified no overall safety issues relating to blood clots — which have been an area of concern for some regulators in Europe.

However, when AstraZeneca released its statement on the trial results on Monday, some of the data and conclusions about the vaccine’s efficacy surprised the DSMB, according to Dr Anthony Fauci, who heads the NIAID.

“The DSMB, when they saw that press release, got concerned and wrote a rather harsh note to them, copied to me, saying that in fact they felt that the data . . . were somewhat outdated and might in fact be misleading a bit and wanted them to straighten it out,” Fauci told ABC News.

Fauci stressed that the AstraZeneca jab was “very likely a very good vaccine” adding that it was “unfortunate” this had happened as it might add to vaccine hesitancy.

“The data really are quite good, but when they put it in the press release, it wasn’t completely accurate,” he said.

Shares in AstraZeneca fell at least 1.5 per cent shortly after the London stock exchange opened on Tuesday morning.

More than 32,000 volunteers took part in the latest trial, mostly in the US, but also in Chile and Peru. A fifth of the participants were aged over 65.

It had been seen as a significant opportunity for the drugmaker to reverse its fortunes in the US, though the NIAID warning has damped those hopes.

Stephen Evans, a pharmacoepidemiology professor at the London School of Hygiene and Tropical Medicine, noted DSMBs sometimes disagreed with investigators over the interpretation of trial results. But he said: “It is usually done in private, so this is unprecedented in my opinion.”

The AstraZeneca shot — easy to store and transport, and currently sold at cost — forms the backbone of many vaccination strategies globally, with the EU, the US, and the World Health Organization-led procurement effort Covax having placed large orders of the jab.

The UK has also given it to millions of people, with real-world evidence suggesting a significant effect in cutting deaths and hospitalisations related to Covid-19.

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