AstraZeneca was in part to blame for the delay, said the people with knowledge of the F.D.A. talks. The company was slow to provide the F.D.A. with all the data that regulators wanted to review before allowing the trial to start up again.

In late October, after 47 days, the F.D.A. cleared the U.S. trial to restart.

It was now even further behind the late-stage trials that Pfizer and Moderna had begun in the United States about a month before AstraZeneca’s trial started.