Pfizer Inc. and BioNTech SE reached a preliminary agreement to supply 200 million doses of their experimental COVID-19 vaccine to the European Union -- the biggest initial order yet for the U.S.-German partners.

The European Commission has concluded exploratory talks and will now begin contract negotiations, according to a statement Wednesday. The deal would include an option for another 100 million doses.

The EU didn’t disclose financial terms. Pfizer and BioNTech previously agreed to supply 120 million doses to Japan. The U.S. in July reached a deal to pay US$2 billion for an initial 100 million doses, with an option for 500 million more.

The companies have said they’re on track for regulatory review as soon as October. They’ve enrolled more than 28,000 of the 30,000 participants needed for a massive clinical trial to show the vaccine works and secured sign-off this week from regulators to expand the study to Germany.

Including study sites in Europe, “and now especially in Germany, is aimed at supporting an approval in Europe,” BioNTech Chief Executive Officer Ugur Sahin said in a previous statement.

The companies’ vaccine relies on a new technology that prompts the body’s own cells to produce the virus proteins needed to trigger the immune system.

The EU previously announced preliminary plans for vaccine supply arrangements with developers including Moderna Inc., Johnson & Johnson, AstraZeneca Plc with the University of Oxford, Sanofi and partner GlaxoSmithKline Plc, as well as CureVac NV.