That could make it more difficult for European officials to demand faster shipments. But Belgian courts, like many in continental Europe, consider not only the wording of a contract, but also its genesis, which could work to the advantage of the European Union.

“A European judge looks at the good faith or lack of it that the parties brought into the contract, at the way in which the contract was negotiated, the general atmosphere,” said Professor Van Calster. “I think that the commission probably hopes that the judge will be able to determine that AstraZeneca, in particular, has failed to supply a number of vaccines which they could have reasonably supplied to the European Union.”

He said that the court could rule that AstraZeneca must surrender a certain number of doses, but that the outcome was uncertain.

AstraZeneca said on Monday that it “regrets the European Commission’s decision to take legal action over the supply of Covid-19 vaccines,” describing the lawsuit as “without merit.”

It said that it would deliver almost 50 million doses to the bloc by the end of April, and that it had “fully complied with the advance purchase agreement with the European Commission and will strongly defend itself in court.”

Stefan de Keersmaecker, a spokesman on health issues for the European Commission, said that the bloc had begun legal action because it believed the purchase agreement had been breached.

negotiating a contract with Pfizer for 1.8 billion doses over the next two years.

Ursula von der Leyen, the president of the European Commission, told The New York Times on Sunday that AstraZeneca had squandered the bloc’s trust.

“The noncommunication of AstraZeneca of the problems made it like pulling a chewing gum, because you never knew what was going on,” she said. Ms. von der Leyen added that the delivery shortfalls were too steep to ignore.

“At the moment, the company has a delay in delivering 200 million doses of vaccine by the end of the second quarter,” she said. “The number speaks for itself.”

AstraZeneca’s vaccine is being widely used across Europe, though some countries have curbed its use in younger people because of the risk of very rare blood clots. The European Medicines Agency, the bloc’s drug regulator, said this month that the shot’s benefits still outweighed its risks, but that it should carry a warning.

The vaccine is not yet authorized for use in the United States. American officials have given a few million doses that were manufactured there to Canada and Mexico.

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