The European Union agreed on Saturday to a massive contract extension with Pfizer-BioNTech for a potential 1.8 billion COVID-19 vaccine doses through 2023.

EU Commission President Ursula von der Leyen tweeted that her office "has just approved a contract for a guaranteed 900 million doses (+900 million options)."

The new contract, which has the backing of the EU member states, will entail not only the production of the vaccines, but also making sure that all the essential components should be sourced from the EU.

The European Commission currently has a portfolio of 2.3 billion doses from half a dozen companies. "Other contracts and other vaccine technologies will follow,'' von der Leyen said in a Twitter message.

Saturday's announcement underscores the confidence the EU has shown in the technology used for the Pfizer-BioNTech vaccine, which is different from that behind the Oxford-AstraZeneca vaccine.

The active ingredient in the Pfizer-BioNTech shot is messenger RNA, or mRNA, which contains the instructions for human cells to construct a harmless piece of the coronavirus called the spike protein. The human immune system recognizes the spike protein as foreign, allowing it to mount a response against the virus upon infection.

America's Pfizer and Germany's BioNTech have already said that they would provide the EU with an extra 50 million doses in the 2nd quarter of this year, making up for faltering deliveries of AstraZeneca. In contrast to the oft-criticized Anglo-Swedish AstraZeneca, von der Leyen has said that Pfizer-BioNTech is a reliable partner that delivers on its commitments.