It is the biggest deal to date under Operation Warp Speed, intended to accelerate the development, manufacturing, and distribution of coronavirus vaccines, therapeutics, and diagnostics.

Pfizer and BioNTech, which are developing the drug together, said in statements that the American people would receive the future vaccine "for free" in line with the Trump administration's pledge.

Under the agreement, the US government has placed an initial order for 100 million doses to be delivered if regulatory approval is granted.

The US government also has an option to purchase as many as 500 million additional doses from the two firms.

BioNTech and Pfizer have narrowed their vaccine candidates down to two frontrunners and are waiting for the green light to begin a mass trial involving 30,000 healthy volunteers, which may happen later this month.

If the studies are successful, they expect to receive some form of emergency approval as early as October 2020.

Earlier this month, they announced that early results showed their lead candidate produced neutralizing antibodies in humans at or above the levels observed in recovered COVID-19 patients.

This was accomplished with relatively low doses and caused side effects that were mild to moderate but transient, which is considered normal.