Pfizer and BioNTech will supply the US with an additional 100 million doses of the COVID-19 vaccine under a new agreement.

The drug makers said on Wednesday they expect to deliver all the doses by July 31.

Pfizer already has a contract to supply the United States with 100 million doses of its vaccine, which requires two doses per patient.

Under the $US2 billion ($A2.6 billion) deal, the companies will deliver at least 70 million of the additional doses by June 30, with the remaining 30 million to be delivered no later than July 31.

The government has the option to acquire up to an additional 400 million doses.

Health and Human Services Secretary Alex Azar said in a statement that the latest deal can give people confidence “that we will have enough supply to vaccinate every American who wants it by June 2021”.

Pfizer’s vaccine was the first to gain approval from the Food and Drug Administration and initial shipments went to states last week.

It has now been joined by a vaccine from Moderna, which was developed in closer co-operation with scientists from the National Institutes of Health.

Moderna’s vaccine comes under the umbrella of the government’s own effort, which is called Operation Warp Speed.

That public-private endeavour was designed to have millions of vaccine doses ready and available to ship once a shot received FDA approval.

The deal with Pfizer moves the nation closer to the goal of vaccinating all Americans.

A law dating back to the Korean War gives the government authority to direct private companies to produce critical goods in times of national emergency.

Called the Defense Production Act, it is expected to be invoked to help Pfizer secure some raw materials needed for its vaccine.

“With these 100 million additional doses, the United States will be able to protect more individuals and hopefully end this devastating pandemic more quickly,” Pfizer CEO Albert Bourla said.

“We look forward to continuing our work with the US government and healthcare providers around the country.”

The vaccine from Pfizer and German pharmaceutical BioNTech immediately raised hopes of taming a pandemic that has killed nearly 320,000 people in the US and hobbled much of the national economy.