The United States has authorised Pfizer Inc's antiviral COVID-19 pill for at-risk people aged 12 and above, making it the first oral treatment and which can be taken at home, providing a potentially important tool in the fight against the fast-spreading Omicron variant.

Pfizer's antiviral regimen, Paxlovid, was nearly 90 per cent effective in preventing hospitalisations and deaths in patients at high risk of severe illness, according to data from the company's clinical trial. Recent lab data suggests the drug retains its effectiveness against Omicron, Pfizer said on Wednesday.

Pfizer raised its 2022 production projections to 120 million courses of treatment from 80 million and said it was ready to start immediate delivery in the US. The treatment's two-drug regimen includes a new medicine and a second older antiviral called ritonavir.

"Paxlovid's approval is a major milestone that marks another step towards making COVID-19 a much more manageable infection," said Amesh Adalja, a senior scholar at the Johns Hopkins Institute for Health Security.

"There are two key issues, however, that remain: It will be scarce in the coming weeks and its optimal use requires prompt diagnosis, which can be difficult with the continual testing problems that plague us," Adalja added.

Pfizer has said it has 180,000 treatment courses ready to ship this year. The US government's contract for 10 million courses of the drug is priced at $US530 ($A740) per course.

The Food and Drug Administration's decision to issue emergency authorisation for the treatment comes as the US combats a surge in cases driven by the Omicron variant, with President Joe Biden announcing plans for more federal vaccination and testing sites.

The Omicron variant, first identified in southern Africa and Hong Kong in November, has spread across the world and now constitutes over 70 per cent of new cases in the United States, according to data from the US Centers for Disease Control and Prevention.

The FDA said it authorised Paxlovid for emergency use for the treatment of mild-to-moderate disease in adults and children 12 years and older, who are at high risk for progression to severe COVID-19.

The drug is available by prescription only and should be initiated as soon as possible after diagnosis and within five days of symptom onset, the FDA said.

The pills are meant to be taken every 12 hours for five days beginning shortly after the onset of symptoms.

Meanwhile, the US Supreme Court on Wednesday agreed to take up disputes over the Biden administration's nationwide vaccine-or-testing mandate for large businesses and a separate vaccine requirement for healthcare workers.

The brief court order said the court will hear oral arguments on January 7 in the two cases.