The single-shot Johnson & Johnson vaccine is highly effective in preventing severe COVID-19, including the South African and Brazil variants, new documents released by the US Food and Drug Administration showed Wednesday.

In large clinical trials, the vaccine efficacy against severe disease was 85.9 per cent in the United States, 81.7 per cent in South Africa, and 87.6 per cent in Brazil.

An independent panel of the Food and Drug Administration will meet to discuss its merits on Friday and an emergency use authorisation is likely to follow soon after.

That would bring a third vaccine into the fight against the outbreak in the United States, the world's hardest-hit country where more than 500,000 people have lost their lives. 

Experts see the J&J vaccine as a vital tool, even though its efficacy against moderate COVID-19 is lower than that demonstrated by the Pfizer and Moderna shots that have already received authorisation.

"The vaccine was effective in preventing COVID-19 using a less restrictive definition of the disease and for more severe disease, including COVID-19 requiring medical intervention, considering all cases starting 14 days after vaccination," the new FDA summary said.

"Although a lower efficacy overall was observed in South Africa, where there was a predominance of B.1.3.5 lineage during the time period of this study, vaccine efficacy against severe/critical COVID-19 was similarly high across the United States, South Africa, and Brazil," it added.

The J&J vaccine uses a common-cold causing adenovirus, which has been modified so that it can't replicate, to carry the gene 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.

The fact that it requires only one dose, and that it can be stored at fridge temperature rather than in freezers like the Pfizer and Moderna shots, gives it an operational advantage.