The European Union has sealed a deal with Johnson & Johnson to supply up to 400 million doses of its potential COVID-19 vaccine, as the bloc builds up stocks amid a global scramble to secure shots.

The deal announced on Thursday by the European Commission is its third advance purchase contract with makers of COVID-19 vaccines after deals with AstraZeneca and Sanofi , bringing the number of doses secured by the EU for its population of 450 million to 1.1 billion.

Under the terms of the deal, the 27 EU states will be able to order up to 400 million doses of the potential vaccine after it is authorised by the EU medicine regulator.

To secure the vaccines, the EU made an undisclosed downpayment to J&J, which confirmed the deal in a statement in which it reiterated plans to allocate up to 500 million additional doses to poorer countries from mid-2021.

EU states would pay the full price only when they order it. The price and liability conditions are confidential.

The J&J deal follows supply contracts for 400 million doses of the potential vaccine being developed by AstraZeneca and for 300 million doses of the shot being trailed by a partnership between Sanofi and GlaxoSmithKline.

The Commission has also said publicly that it was in advanced talks to secure vaccines being developed by Moderna , CureVac and a partnership between Pfizer and BioNTech, which if they are confirmed would give the EU a total supply of nearly 2 billion doses.

The EU is in talks with Novavax for a seventh vaccine, a senior EU source told Reuters last month .

If it strikes seven deals, the EU would be ahead of Britain and the United States, which each have concluded six supply contracts so far.

The J&J vaccine, which is being developed by its subsidiary Janssen, is based on vector technology, the same used by AstraZeneca. Sanofi's is a protein-based jab.