London CNN Business  — 

The European Union is pivoting to the Pfizer/BioNTech coronavirus shot with a record agreement to buy up to 1.8 billion doses, as court proceedings in its lawsuit against another major vaccine maker, AstraZeneca, began in Brussels on Wednesday.

The agreement with Pfizer, which is yet to be finalized in an official contract, would be the world’s biggest single deal for a Covid-19 vaccine to date. European Commission President Ursula von der Leyen announced the landmark purchase on Friday during a visit to a Pfizer manufacturing plant in Puurs, Belgium, saying the vaccines would be delivered through to 2023.

The deal was announced as pressure mounts on developed nations to stop purchasing more doses than their populations need to ensure there are enough for the rest of the world.

An account of the negotiations published Wednesday by the New York Times, based on interviews with von der Leyen, Pfizer CEO Albert Bourla and other experts and officials, painted a picture of personal diplomacy between the Pfizer and EU chiefs. It related how the pair exchanged regular text messages and calls over months until it became clear Pfizer could supply the EU with far more doses than the 300 million it had already agreed to.

“Multiple leaders of the world, they would reach out to me, from presidents or prime ministers and kings, and general secretaries of organizations,” Bourla told the New York Times, explaining that such conversations were not uncommon.

Von der Leyen described the difficulties she faced as a leader as it became clear AstraZeneca was not going to deliver on its targets.

“I knew that the upscaling of the deliveries would have a slow start by nature in the beginning, and therefore, I also knew the first quarter was going be tough,” she said.

“I did not expect it to be as tough, because we did not include the possibility that AstraZeneca would reduce deliveries by 75%. That was a heavy setback.”

Von der Leyen said that the Pfizer deal would include an initial 900 million doses with the option for an additional 900 million, according to the New York Times report. The European Commission did not immediately confirm that detail to CNN.

A European Commission spokesperson would not comment on the details outlined in the New York Times report, instead referring CNN to von der Leyen’s remarks on Friday, “where she addressed the importance of a productive relationship between the European Commission and the various stakeholders involved in our vaccine strategy.”

But her confidence appeared to have returned on Friday as she announced the Pfizer (PFE) deal, and sought to repair the image of the European Union as a world leader in global health.

“[The contract] will secure the doses necessary to give booster shots to increase our immunity against the virus. It will provide vaccines adapted to escape variants that no longer respond to the vaccines. And it should enable us to vaccinate, if necessary and safe, children and teenagers. And it will consolidate Europe’s leadership in mRNA technologies,” she said, referring to the technology used in vaccines from Pfizer and Moderna.

The European Commission announced Monday it was suing AstraZeneca over an alleged breach of its vaccine supply contract, in a dramatic escalation of a months-long dispute over delivery delays that hampered the rollout of shots across much of the continent.

The 27 nations of the EU had ordered 300 million doses of the Covid-19 vaccine from AstraZeneca to be delivered by the end of June, with an option to purchase an additional 100 million. But deliveries of the vaccine repeatedly fell millions of doses short. The British-Swedish drugmaker has said it will deliver 100 million doses by the end of June, a third of what it originally set out to in its contract with the bloc.

Outside the court in Brussels, a lawyer representing AstraZeneca, Hakim Boularbah, said: “The only statement I can make is that AstraZeneca deeply regrets the decision of the European Commission to take this action to court. They hope the dispute will be resolved as soon as possible.”

The company has dismissed allegations it was in breach of contract several times. Its CEO Pascal Soriot in January highlighted that its contract is based on the company’s best efforts to deliver to schedule and that targets are not legally binding.

The full, unredacted contract between the European Commission and AstraZeneca – first published by Italian broadcaster RAI – includes a clause that appears to protect the company from legal action for delays in delivery. A court may rule, however, on whether the company did indeed make its “best reasonable efforts” to make good on its targets, as its contract states.

A lawyer for the Commission, Rafaël Jafferali, said outside the courtroom Wednesday: “We made our case in court. We explained the situation. Our comments are for the court.”

A court spokeswoman told CNN that the EU wants the company to catch up on its deliveries to the bloc.

The next hearing is scheduled for May 26. The judge is expected to take three to six weeks to come back with a ruling.

The lawsuit is the latest in a string of problems for AstraZeneca (AZN). The company came under scrutiny over the way it presented its clinical trial data in Europe in the earlier stages of the pandemic, and more recently in the United States.

Reports of a rare but sometimes fatal blood clotting condition following use of the vaccine in younger adults, mostly women, has prompted some countries to restrict its use to only older members of their populations.

AstraZeneca will release its first quarter financial results on Friday.