The agreement allows the US government to acquire an additional 500 million doses, the Department of Health and Human Services and the Department of Defense said.

The Trump administration has agreed to spend billions of dollars for the development and procurement of a potential vaccine. The administration launched 'Operation Warp Speed' — a joint HHS and Department of Defense program — to accelerate the development of coronavirus vaccines, treatments and diagnostics.

More than 150 coronavirus vaccines using a variety of technologies are in development globally, with some two dozen already in human trials. Governments have signed deals with drugmakers to secure the supply of various vaccine candidates.

The aim is to produce vaccines that can end the pandemic by protecting billions of people from infection or severe illness.

Pfizer and BioNTech's vaccine candidate is among those that are set to be tested in a large trial. The vaccine has shown promise in early-stage small studies in humans.

Pfizer will deliver the doses if the product receives Emergency Use Authorization or licensure from the US Food and Drug Administration, after completing demonstration of safety and efficacy in a large Phase 3 clinical trial.

The companies said they expect to be ready to seek some form of regulatory approval as early as October, if the ongoing studies are successful.

Pfizer and BioNTech currently expect to manufacture up to 100 million doses globally by the end of 2020, and potentially more than 1.3 billion doses by the end of 2021, subject to final dose selection from their clinical trial.

Pfizer's shares rose 4%, while BioNTech's US-listed shares were up about 6% before the bell.