The U.S. also preordered 300 million doses in a deal valued well over $1 billion. AstraZeneca hopes to submit trial data to U.S. regulators as soon as February, with emergency-use authorization potentially coming in March or April.

Dr. Soriot, a French and Australian citizen, has said the EU was behind others in locking in its orders. He has said the company is roughly three months behind fixing manufacturing "glitches" in Europe it already worked out in the U.K. and elsewhere.

The EU has said AstraZeneca signed a contract pledging to deliver tens of millions of doses in early 2021 including from the firm's U.K. plants if necessary. The EU has set aside EUR336 million to prepay for doses and help the company meet its delivery pledges.

"We reject the logic of first-come, first-served," European Health Commissioner Stella Kyriakides responded Wednesday. "That may work in the neighborhood butcher's, but not in contracts, and not in our advance-purchase agreements."

Well before AstraZeneca's manufacturing problems, the AstraZeneca-Oxford partnership has been marred by missteps that sowed doubts about the effectiveness of the vaccine, stirring tensions between the university and the drugmaker.

When releasing late-stage human trial data, the two said the vaccine showed to be between 62% and 90% effective in preventing Covid-19 symptoms. They later clarified that the 90% number was from a small subset of trial participants, all of whom were under age 55 and received a nonstandard, smaller first dose. That regimen has been rejected by regulators in the U.K.

A shortage of data showing the vaccine's effectiveness in the elderly could be a sticking point with European regulators, who have been weighing whether to approve the vaccine for anyone over 65, according to people familiar with the matter. The European Medicines Agency, which approves drugs for the EU much like the Food and Drug Administration does in the U.S., is expected to rule on the AstraZeneca vaccine Friday. The U.K. approved the shot's use for adults 18 and older, without old-age restrictions.

On Thursday, advisers to the German government recommended the vaccine be used only in people age 64 or younger, pending more data on efficacy in the elderly.

--Luciana Magalhaes in São Paulo, Rhiannon Hoyle in Sydney and Joseph Walker in New York contributed to this article.

Write to Jenny Strasburg at jenny.strasburg@wsj.com and Laurence Norman at laurence.norman@wsj.com

(END) Dow Jones Newswires

01-28-21 1727ET