WASHINGTON: British-Swedish drugmaker AstraZeneca on Wednesday revised down by three percentage points the effectiveness of its Covid vaccine after American authorities raised concerns that results reported from its US trial were outdated.

The company now says its vaccine is 76 rather than 79 percent effective at preventing any kind of symptomatic Covid.

It remains 100 percent effective against severe Covid, it added.

The move came after an independent panel of experts appointed to supervise the trial expressed concern that AstraZeneca had failed to include updated data in its initially released figure.

The US National Institutes of Health then issued a highly unusual statement asking AstraZeneca to work with the panel and issue a new press release.

"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," said Mene Pangalos executive vice president of biopharmaceuticals research and development.

The new figure is based on 190 people who fell sick among 32,449 trial participants across the United States, Peru and Chile, two-thirds of whom received the vaccine while the rest received a placebo.

The previous figure was based on 141 cases, with a cut-off in mid-February.

The company added there are 14 additional or probable Covid cases left to be adjudicated.