The European Commission said it expects to seal the world's biggest vaccine supply deal within days, securing up to 1.8 billion doses of Pfizer's COVID-19 vaccine for the next few years as a debate rages over unfair access to shots for the world's poorest people.

The vaccines from the U.S. drugmaker and its German partner BioNTech would be delivered over 2021-2023, Commission President Ursula von der Leyen said during a visit to Pfizer's vaccine plant in Puurs, Belgium.

The agreement, which is to include 900 million optional doses, would be enough to inoculate the 450 million EU population for two years and comes as the bloc seeks to shore up long-term supplies.

This is the third contract to be agreed by the bloc with the two companies, which have already committed to supplying 600 million shots of the two-dose vaccine this year under two previous contracts. Brussels is aiming to inoculate at least 70 percent of EU adults by the end of July.

The move comes as the Commission looks to sever ties with AstraZeneca after the drugmaker slashed its delivery targets due to production problems. On Friday it was moving closer to take legal action against the British-Swedish pharmaceutical company.

An EU official said the supply deal with Pfizer was agreed in principle but that both sides needed a few days to iron out final terms.

"We will conclude in the next days. It will secure the doses necessary to give booster shots to increase immunity," von der Leyen said at a news conference with Pfizer boss Albert Bourla.

Pfizer has scrambled to boost output in recent months at its U.S. and Belgian plants to meet growing demand.

(With input from agencies)