It’s another setback for the AstraZeneca COVID-19 vaccine which has been dogged by doubts over its efficacy, dosing regime and possible side effects.
A successful large-scale trial of the shot in the U.S. is now being questioned by the Data Safety Monitoring Board, an independent committee overseeing the trial.
It has expressed concern that AstraZeneca may have included "outdated information" from that trial, which then may have provided an incomplete view of the efficacy data.
The concerns throw into question whether the British drugmaker can seek U.S. emergency use authorization for the vaccine in the coming weeks as planned.
Just one day ago interim data from the trial had shown better-than-expected results.
The vaccine developed with Oxford University was 79% effective in preventing symptomatic illness in the large trial of 32,000 people that also took place in Chile and Peru.
According to the data it was also 100% effective against severe or critical forms of the disease and hospitalization.
And posed no increased risk of blood clots.
It has been granted conditional marketing or emergency use authorization in more than 70 countries.
After briefly halting its use, many European countries have resumed using the shot.
Several country leaders are also taking the vaccine to boost confidence.
In the U.S. the U.S. National Institute of Allergy and Infectious Diseases is now urging the company to review the efficacy data.
AstraZeneca did not immediately respond to a Reuters request for comment.