A US panel of independent experts voted unanimously Friday in favor of recommending Johnson & Johnson's one-dose Covid-19 shot for emergency approval, clearing the way for a third vaccine to soon begin shipping in the world's hardest hit country.
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 aren't binding, they are usually followed.
An emergency use authorization (EUA) will likely follow, probably in the coming days, making the J&J vaccine the third to be greenlighted in the United States after Pfizer's and Moderna's were provisionally approved in December.
"We've all seen the news about (the) Johnson and Johnson vaccine today -- (the) third, safe effective vaccine," President Joe Biden said from Houston shortly after the vote.
"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.
"Janssen's vaccine candidate will play a critical role in the global efforts to fight Covid-19," Johan Van Hoof, global head of vaccines research and development at J&J's Janssen subsidiary, had told the meeting, emphasizing the shot's high efficacy against severe Covid-19, 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."
- Placebo crossover -
In a global trial of almost 40,000 people, the vaccine's efficacy against severe disease was 85.4 percent, but it fell to 66.1 percent 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 percent 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.
Van Hoof said that if authorized, J&J would apply to amend their trials so that people who received the placebo could now receive the vaccine.
The idea is to compare relative efficacy between people who were vaccinated earlier against later, to see if immunity wanes over time.
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.
- Anaphylaxis case -
During J&J's presentation, company scientist Macaya Douoguih revealed that a case of anaphylaxis occurred in a health worker in South Africa -- the first time this has happened for the shot.
Some rare anaphylaxis cases have been reported for the Pfizer and Moderna shots.
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).
Seven people died from Covid-19 in the placebo group, and none in the vaccine group.
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 expedite that timeline.
The J&J vaccine uses a common-cold causing adenovirus, which has been genetically modified so that it can't 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.