BRUSSELS, Sept 23 (Reuters) - Brussels is close to wrapping up preliminary talks with U.S. vaccine maker Novavax for its potential COVID-19 shot, an EU source involved in the negotiations said, as the bloc ramps up its push in the global race to secure supplies.

The move is meant to expand to seven the portfolio of vaccines the 27-country bloc, with a population of 450 million, wants to secure. It would bring doses potentially available to EU countries to more than 2 billion.

"We are at an advanced stage in talks with Novavax," the EU senior official said, who declined to be identified as negotiations are confidential.

The number of doses the EU could secure is still under discussion, the official said, noting that it was likely to be at least 100 million.

A spokesman for the EU executive commission, which is co-leading the talks with vaccine makers, declined to comment. Novavax had no immediate comment.

The EU has already secured 400 million doses of the potential vaccine being developed by AstraZeneca and Oxford University, and 300 million doses of the potential shot being trialled by Sanofi and GlaxoSmithKline.

It is also in talks with Johnson & Johnson, Moderna , CureVac and a partnership of Pfizer and BionTech for nearly 1.3 billion additional doses of their vaccine candidates.

The seventh vaccine would add to the EU portfolio a second protein-based jab, the same technology used by Sanofi and GSK. The EU is also securing shots based on messenger RNA (mRNA) and vector technologies.

"We want to split the risks," the official said, as it is not certain which, if any, of the vaccines under development will prove effective and safe.

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.

Novavax has already reached supply deals with the United States, Britain, Japan and Canada. It is also one of the potential vaccines covered by the WHO-led procurement facility COVAX.

Crucial for the deals to be finalised is the topping up of an EU emergency fund, which has so far made available 2 billion euros ($2.3 billion) for vaccine purchases. At least another 750 million euros is needed to cover the seven deals, the EU official said.

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(Reporting by Francesco Guarascio; Editing by Josephine Mason and Jan Harvey)