A single-shot Covid jab moved a step closer to being approved in the US after a panel voted in favour of authorising it for emergency use.
The Johnson & Johnson vaccine is expected to be approved by the FDA in the US, amid hopes the UK will follow suit.
Britain has ordered 30 million doses of the jab, and it could be a gamechanger in speeding up the rollout in coming months.
All three vaccines approved in the UK - made by Pfizer, Oxford University/AstraZeneca and Moderna require two doses, which are currently being administered 12 weeks apart.
The panel of FDA advisers - consisting of doctors, infectious disease experts and medical researchers - voted unanimously that the benefits of the vaccine outweigh its risks in people age 18 and over.
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J&J has said it will be able to ship three million to four million doses of its vaccine next week if it receives the emergency use authorization (EUA), as expected.
Senior White House adviser Andy Slavitt tweeted that the FDA would meet to finalize the EUA on Saturday.
"A third safe and effective vaccine is very welcome news," he said.
More than 50 million vaccine shots have been administered nationwide, a milestone highlighted by President Joe Biden on Thursday.
Panel member Jay Portnoy, Professor of Pediatrics UMKC School of Medicine, said the race was on to stop the pandemic before new virus variants cause further disease.
"The fewer people who are infected with the virus, the less opportunity it has to emerge as a more virulent strain.
"So we're in a hurry to get this vaccine out."
The head of the U.S. Centers for Disease Control and Prevention on Friday expressed concern that a recent decline in US Covid-19 cases may be stalling.
In a 44,000-person trial, the J&J vaccine overall was 66 per cent effective at preventing moderate-to-severe cases of Covid-19 compared with a placebo.
Two-dose vaccines made by Pfizer Inc with BioNTech SE and Moderna Inc were about 95 per cent effective.
Experts cautioned against drawing too much of a distinction between the vaccines because the trials had somewhat different goals, and importantly, the Pfizer/BioNtech and Moderna studies were conducted before the more transmissible virus variants had emerged in South Africa, Brazil and the UK.
J&J's trial was underway when the new variants were circulating and even prevalent in some countries where it was tested.
Still, its vaccine was 100 per cent effective at preventing hospitalisations 28 days after vaccination and there were no Covid-19 deaths among those who received the shot.
During the meeting, a J&J vaccine executive said the company was developing a second-generation vaccine that would target the concerning South African variant and will be ready to start Phase I trials by this summer.
The J&J vaccine can be stored in normal refrigerator temperatures, making distribution easier than for the Pfizer/BioNTech and Moderna shots that use messenger RNA technology and are shipped frozen.
