WASHINGTON, United States (AFP)— A US panel of independent experts voted unanimously Friday in favour 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.

Administration officials said this week they hoped to deliver three to four million doses next week.

The vote followed a daylong, live streamed virtual meeting to decide if the known benefits of the drug outweigh its known risks for use in people 18 and over -- an exercise in transparency unparalleled among advanced countries, that 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 R&D at J&J's Janssen subsidiary, had told the meeting, emphasizing the shot's high efficacy against severe COVID-19, including against new, concerning 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 per cent, but it fell to 66.1 per cent 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 per cent against symptomatic COVID-19.

It is also possible the experts might seek an age cap, given that only around 1,500 participants in the trial were over the age of 75.

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 panellists who felt people who got one dose might feel short-changed if two doses ends up proving more effective.