A single-shot coronavirus vaccine is effective against new variants and could be approved for distribution this week, US officials have said.
The Johnson & Johnson vaccine is 85.9 per cent effective at preventing severe illness and 66 per cent protective against moderate cases, according to experts from the US Food and Drug Administration.
Crucially, analyses of different demographic groups revealed no marked differences across age, race, or people with underlying conditions.
White House Coronavirus Response Coordinator Jeff Zients told reporters that if authorised, the federal government would seek to distribute three to four million doses next week.
"Johnson & Johnson has announced it aims to deliver a total of 20 million doses by the end of March," he said, adding the government was trying to speed up the delivery of the contracted 100 million doses, which the company has promised by the end of June.
A third vaccine is seen as a vital means to ramp up the immunisation rate in the United States, where more than 500,000 people have lost their lives to the virus.
Some 65 million people in America have so far received at least one shot of either the Pfizer or Moderna vaccines - but the Johnson & Johnson vaccine requires just one dose, and is stored at fridge temperatures.
The UK has ordered 30 million doses, with the option for 22 million more.
The Johnson & Johnson vaccine uses a common-cold causing adenovirus, which has been modified so that it can't replicate, to carry the DNA for a key protein of the coronavirus into human cells.
This makes those cells produce that protein, which in turn trains the human immune system should it encounter the real virus.
Other adenovirus vector vaccines against Covid-19 include those made by AstraZeneca-Oxford and Russia's Sputnik V.