AstraZeneca has 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of its Covid vaccine after American authorities raised concerns that results reported from its US trial were outdated.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% effective at preventing any kind of symptomatic Covid.
It remains 100% 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 roll out 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 US, 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.
Depending on whether these occurred in the vaccine or placebo group, the final efficacy figure could go up or down a few percentage points.
The company added the vaccine efficacy in people 65-years-old and over was 85%, but the statistical range for this figure, anywhere between 58-95%, make it less meaningful and the sample size was not mentioned.
Eight cases of severe Covid occurred, all in the placebo group.
AstraZeneca was an early frontrunner in the global race to develop a Covid vaccine.
It was heavily favoured by the US, which ordered 300m doses, more than its first orders for Moderna's and Pfizer's shots.
But a series of communications blunders eroded US agencies' confidence.
The US now has three authorised vaccines, Moderna, Pfizer and Johnson & Johnson, and should have enough supply to more than cover the adult population by the end of May.
However, authorisation by the Food and Drug Administration, considered the gold standard regulator,would go a long way to calm global doubts over the AstraZeneca vaccine.
