A US panel of independent experts has voted unanimously in favour of recommending Johnson & Johnson's one-dose Covid-19 shot for emergency approval.
The decision clears the way for a third vaccine to soon begin being distributed in the country hardest hit by the global pandemic.
The committee's 22 members were convened by the Food and Drug Administration and included leading scientists as well as consumer and industry representatives.
Although their recommendations are not binding, they are usually followed.
An emergency use authorisation (EUA) will now likely follow, probably in the coming days, making the J&J vaccine the third to be licensed in the United States after the Pfizer-BioNTech and Moderna inoculations were provisionally approved in December.
Reacting to the vote from Texas, where he is visiting in the wake of a deadly winter storm, President Joe Biden said: "We've all seen the news about the Johnson & Johnson vaccine today, the third, safe effective vaccine.
"We're going to use every conceivable way to expand manufacturing of the vaccine and make even more rapid progress in getting shots to people's arms."
Administration officials said this week they hoped to deliver three to four million doses next week.
The recommendation followed a day-long, live-streamed virtual meeting to decide if the known benefits of the drug outweigh its known risks for use in people aged 18 and over - which gave the public an inside look at the details of the scientific debate.
Johan Van Hoof, global head of vaccines research and development at J&J's Janssen subsidiary, had told the meeting: "Janssen's vaccine candidate will play a critical role in the global efforts to fight Covid-19," emphasising the shot's high efficacy against severe Coronavirus, including against new variants.
He also stressed the fact that it was proven to work with a single shot, and could be stored long term at fridge temperatures, which "offers logistical and practical advantages."
In a global trial of almost 40,000 people, the vaccine's efficacy against severe disease was 85.4%, but it fell to 66.1% when including moderate forms of the disease.
This makes it slightly less protective than Pfizer and Moderna's two-shot regimens, which both have an efficacy of around 95% against symptomatic Covid-19.
Nevertheless, experts say J&J's shot will bring to bear a powerful new weapon against the outbreak in the United States, where more than 500,000 people have died.
The company is also planning to soon begin testing its vaccine on children.
It is separately studying whether two doses might be more protective than one, which raised concerns among some panelists who felt people who got one dose might feel short-changed if two doses ends up proving more effective.
In almost 44,000 people studied for safety issues, the vaccine was well tolerated, with typical side-effects like injection-site pain, headache, fatigue and muscle pains.
There were a handful of cases of people who developed hives, which could have been linked to the shot.
The FDA also said long-term monitoring was needed to determine whether the vaccine was linked to a small number of cases of potentially dangerous clotting, and to tinnitus (ear ringing).
There was a hint, based on preliminary data, that the vaccine might be partly effective against asymptomatic infection.
The company has announced it aims to deliver a total of 20 million doses to the United States by the end of March, with 100 million by June, though the US is pushing to speed up that timeline.
The J&J vaccine uses a common-cold causing adenovirus, which has been genetically modified so that it cannot replicate, to carry the gene for a key protein of the Coronavirus into human cells.
Those cells then produce that protein, which in turn trains the human immune system should it encounter the real virus.
After J&J submitted an application for approval in the US, South Korea began a review of the vaccine.
The South Korean food and drug safety ministry said it would study the vaccine with private advisory panels before it authorises it.
The European Medicines Agency is expected in 12 days time to approve the Johnson & Johnson vaccine for use in the European Union.