AstraZeneca has said it is standing by its coronavirus vaccine in the face of concerns about its US trial results.

The British-Swedish company said that data it published showing the vaccine to be highly effective against Covid-19 were based on an interim analysis, and that it would release new analysis and data “within 48 hours”.

The US National Institute for Allergies and Infectious Diseases raised concerns that the results were out-dated.

“We have reviewed the preliminary assessment of the primary analysis and the results were consistent with the interim analysis,” the company said in a statement. “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.”

The company is seeking approval for the drug from the US Food and Drug Administration. The US trial data, based on 30,000 volunteers, if validated, shows the vaccine to be 79 effective in Covid-19 symptoms, and 100 per cent effective in preventing the most serious effects.

AstraZeneca says it can supply the US with 50 million doses in the first month after FDA approval. It is seen as a potential game-changer, as it is easier and cheaper to store and transport than other vaccines.

The EU approved the vaccine three months ago, and it has come under fire over concerns that it may be related to blood clots. More than a dozen countries, including France, temporarily suspended use of the AstraZeneca vaccine because of isolated cases of blood clots. The World Health Organization and regulators have found no link with the shot.

In the EU the company has struggled with production, delivering only 30 per cent of the doses it had promised the bloc in the first quarter of 2021.

The European Commission on Wednesday will revise trade rules to allow it to shore up its vaccine stock by preventing exports produced in the EU.