NEW YORK - A vaccine from Johnson & Johnson which could be delivered to the UK in the second half of this year is 66% effective against Covid-19, trial results show.

The single-shot vaccine, which has been developed by Johnson & Johnson's pharmaceutical arm Janssen, is 66% effective overall at preventing moderate to severe Covid-19 28 days after vaccination.

The firm said the jab was 85% effective in preventing severe disease "and demonstrated complete protection against Covid-19-related hospitalisation and death as of day 28".

This means that one month after vaccination, no one who received the vaccine was admitted to hospital or died, regardless of which strain of coronavirus they were exposed to.

Overall, the jab worked across multiple variants of coronavirus, including the South African variant which has been worrying scientists.

The new vaccine was tested in a clinical trial involving 43,783 people, during which time 468 Covid-19 cases were recorded.

The level of protection against moderate to severe Covid-19 infection was found to be 72% in the United States arm of the trial, 66% in the Latin American arm and 57% in the South African arm, where a mutant strain of the virus has been dominating.

The overall efficacy from these trials combined was 66%.

The UK has ordered 30 million doses of the vaccine, with the option of 22 million more, with deliveries expected in the second half of this year if the jab is approved.

Health Secretary Matt Hancock tweeted: "This is yet more good news from Janssen on vaccines.

"If this jab is approved this could significantly bolster our vaccination programme, especially as a single-dose vaccine.

"Once the full data has been submitted to the Medicines & Healthcare products Regulatory Agency, they will consider the evidence to determine whether the vaccine meets robust standards of safety, effectiveness & quality.

"We're rolling-out vaccines as quickly as possible across the UK, with more than 7.4 million people given their first dose so far."

Paul Stoffels, chief scientific officer at Johnson & Johnson, said: "The potential to significantly reduce the burden of severe disease, by providing an effective and well-tolerated vaccine with just one immunisation, is a critical component of the global public health response.

"A one-shot vaccine is considered by the World Health Organisation to be the best option in pandemic settings, enhancing access, distribution and compliance.

"Eighty-five per cent efficacy in preventing severe Covid-19 disease and prevention of Covid-19-related medical interventions will potentially protect hundreds of millions of people from serious and fatal outcomes of Covid-19.

"It also offers the hope of helping ease the huge burden placed on healthcare systems and communities."

Earlier, Professor Paul Heath, who headed a vaccine trial for Novavax, which reported its results late on Thursday, said vaccines can be adapted with such speed that scientists should be able to "get ahead" of new strains of coronavirus.

Experts have been deeply worried about the strains found in South Africa and Brazil due to the fact that existing vaccines may not be fully effective against them.

The Novavax data shows its vaccine is 89% effective at preventing Covid-19 in a clinical trial run in the UK, which involved more than 15,000 people aged between 18-84, of whom 27% were older than 65.

More than 50% of cases related to the UK strain of the virus first identified in Kent, with the vaccine offering 86% protection against this strain.

Against the original strain that has circulated since the start of the pandemic, the vaccine was 96% effective.

In a separate South African arm of the trial, where most cases of Covid-19 were the worrying South African strain, the jab was 60% effective in people without HIV.

The UK has secured 60 million doses of the Novavax jab, to be produced on Teesside, with the hope that the MHRA could approve it within weeks.

Prof Heath told BBC Radio 4's Today programme there was reason for optimism that vaccines could keep pace with any new variants that are emerging.

He said: "I think the technology we have, both with this vaccine, the Novavax technology, and the other vaccines, it is such that they can adapt quickly so we can expect to see, if required, new vaccines or bivalent vaccines, where two different strains are joined together in the one vaccine.

"And that now can be done at pace so that we can keep up with these variants should they prove to be difficult to prevent with the vaccine that we have at the moment.

"We've seen for the UK that the UK variant can be successfully prevented with this vaccine, which is great.

"Yes, the South African variant is more difficult... but all the technologies we are now seeing mean that we can adapt very quickly to such new variants and produce new vaccines at pace so that we can keep up, and in fact get ahead of, the virus."

In the clinical trial in South Africa, about one third of people tested positive for previous infection with coronavirus.

Novavax said this data "suggest that prior infection with Covid-19 may not completely protect against subsequent infection by the South Africa escape variant" but said its jab provided significant protection.

Professor Paul Hunter, from the University of East Anglia, said that although the trial suggested lower efficacy against the South African variant, 60% protection "is still a worthwhile level of immunity".

He said the vaccine may also offer additional protection against more severe disease.