AstraZeneca has announced it downgraded the efficacy result of its coronavirus vaccine trial in the United States after health officials questioned the results.

The company adjusted the efficacy rate of its vaccine against Covid-19 symptoms from 79% to 76%, but said the trial results confirm it “is highly effective in adults”.

US health officials had been concerned the trial was using outdated data.

AstraZeneca said it now looked forward to getting US regulatory approval.

The US trials of the AstraZeneca jab had involved more than 32,000 volunteers, mostly in America, but also in Chile and Peru.

In the results announced on March 22, the company said the vaccine was found to be 79% effective at stopping symptomatic Covid-19 disease and was 100% effective at preventing people from falling seriously ill.

On March 23, the National Institute of Allergy and Infectious Diseases said it had been informed by data and safety officials monitoring the trial that information may have been used that provided an “incomplete view of the efficacy data”.

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Dr. Anthony Fauci, the White House’s Chief Medical Advisor, then warned reporters the company would “likely come out with a modified statement”.

AstraZeneca’s revised results now put the vaccine’s overall efficacy at 76% instead of 79%. Among the over 65s, its efficacy rose from 80% to 85% and against severe disease it remains 100% effective.

On March 25, Mene Pangalos, an executive vice president at AstraZeneca, said: “We look forward to filing our regulatory submission for Emergency Use Authorization in the US and preparing for the rollout of millions of doses across America.”

The US had ordered 300 million doses of the Oxford-AstraZeneca vaccine when it emerged as a frontrunner in the global race to immunize people against Covid-19.

However, delays and controversies have seen three other vaccines beat it to a US rollout.