The US government has agreed to pay almost $2 billion for 100 million doses of a potential coronavirus vaccine, German firm BioNTech said on Wednesday.

BioNTech, which is developing the drug with US pharma giant Pfizer, said in a statement that American people would receive the future vaccine "for free" in line with the Trump administration's "commitment for free access for COVID-19 vaccines".

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, BioNTech added.

Labs around the world are racing to produce a vaccine to help end the worst health crisis in over a century. More than 200 candidate vaccines are currently being developed with roughly two dozen at the stage of clinical trials with human volunteers.

BioNTech and Pfizer have narrowed their vaccine candidates down to two frontrunners and waiting for the green light to begin a mass trial involving 30,000 healthy volunteers, which may happen later this month. Subject to successful outcomes and regulatory approvals, Pfizer and BioNTech expect to manufacture up to 100 million doses by the end of 2020, and "potentially more than 1.3 billion doses by the end of 2021".

Their vaccine method relies on using messenger RNA, genetic code that slips into human cells to make them generate antibodies that are specifically tailored to the coronavirus. Trials so far have shown that their treatment triggers "robust" antibody and T cell immune responses against the novel coronavirus, according to BioNTech.

The agreement between the US, Pfizer and BioNTech "increases the odds that we will have a safe, effective vaccine as soon as the end of this year", said US health official Alex Azar.  A BioNTech spokeswoman told AFP that two injections would probably be needed for maximum protection, with the booster shot following seven days after the first injection.

Based on the price paid by the US government, it would, therefore, cost $39 to immunise a person against the deadly virus. Oxford University and AstraZeneca are also working on a vaccine, which they have vowed to make available "at cost price", amounting to roughly $2.90 per unit.