J&J, via its subsidiary Janssen, had already received $456 million in March.

The new money will allow the company to ramp up production so that doses are ready for shipping if and when the drug receives regulatory approval.

The government also has the option to acquire additional doses sufficient to vaccinate 300 million people.

With the latest deal, President Donald Trump's administration has spent $9.4 billion on vaccine agreements, with five companies agreeing to provide at least 700 million doses, according to an AFP tally.

These deals, under Operation Warp Speed, foresee launching production in parallel with clinical trials, with the government taking the financial risk away from the private sector.

The US has also spent billions on building manufacturing sites, on companies that make syringes and vials, and on the development of treatments.

Washington has bought up almost all stock of the antiviral drug remdesivir that will be produced until September. The medicine is the first treatment with proven benefit against COVID-19.

Also Wednesday, Canada signed an agreement for undisclosed financial terms with Pfizer Canada and BioNTech SE for 100 million doses of their experimental vaccine in 2020, and more than one billion in 2021, under "Project Lightspeed."