WASHINGTON • Johnson & Johnson (J&J) has said that its single-dose vaccine was 66 per cent effective in preventing Covid-19 in a large trial against multiple variants across three continents.

In the trial of nearly 44,000 volunteers, the level of protection against moderate and severe Covid-19 varied from 72 per cent in the United States, to 66 per cent in Latin America and just 57 per cent in South Africa, from where a worrying variant has spread.

A high bar has been set by two authorised vaccines from Pfizer-BioNTech and Moderna, which were around 95 per cent effective in preventing symptomatic illness in pivotal trials when given in two doses. Those trials, however, were conducted mainly in the US and before new variants emerged.

J&J's main goal was the prevention of moderate to severe Covid-19, and the vaccine was 85 per cent effective in stopping severe disease and preventing hospitalisation across all geographies and against multiple variants 28 days after immunisation.

That "will potentially protect hundreds of millions of people from serious and fatal outcomes of Covid-19," Dr Paul Stoffels, J&J's chief scientific officer, said in a statement issued yesterday with the results, which were based on 468 symptomatic cases. J&J plans to seek emergency use authorisation from the US Food and Drug Administration next week.

Unlike the Pfizer-BioNTech and Moderna vaccines, J&J's does not require a second shot weeks after the first or need to be kept frozen, making it a strong candidate for use in parts of the world with weak transportation infrastructure and insufficient cold storage facilities.

The announcement comes as US biotechnology firm Novavax said its two-shot Covid-19 vaccine showed an overall efficacy of 89.3 per cent in a major phase three clinical trial in Britain, and remained highly effective against a variant first identified there. But the positive news was partly offset by other results that showed it offered significantly less protection against the South Africa variant, known as B1351, which is spreading rapidly around the world.

Novavax said on Thursday that it began working on new vaccines against emerging strains early this month and expects to select ideal candidates in the coming days, before starting clinical testing in the second quarter of the year.

"NVX-CoV2373 has the potential to play an important role in solving this global public health crisis," said the company's president and chief executive officer Stanley Erck, using Novavax's name for the vaccine.

"We look forward to continuing to work with our partners, collaborators, investigators and regulators around the world to make the vaccine available as quickly as possible."

British Prime Minister Boris Johnson tweeted that the results were good news, adding: "Our medicines regulator will now assess the vaccine, which will be made in Teesside. If approved, we have 60m doses on order."

The vaccine was one of six candidates backed by a US government project formerly known as Operation Warp Speed, which has provided the company with US$1.75 billion (S$2.33 billion).

Unlike the Pfizer, Moderna and AstraZeneca vaccines, which deliver the genetic instructions that make human cells create a key protein of the virus, the Novavax shot injects the protein directly into the body to evoke an immune response.

The company's preliminary analysis indicated that the variant first identified in Britain, B117, was detected in more than 50 per cent of the confirmed cases.

Efficacy by strain was calculated at 95.6 per cent against the original Covid-19 strain, and 85.6 per cent against B117.

But the level of protection was lower in a smaller, mid-stage trial conducted in South Africa, where overall efficacy was 49.4 per cent. But the figure rose to 60 per cent among the 94 per cent of trial participants who were HIV-negative.

Pfizer and Moderna have previously said their vaccines remain effective against the variants, but their studies were based on laboratory research.

REUTERS, AGENCE FRANCE-PRESSE