The Oxford University vaccine is 70% effective, trials have shown.
The trials also showed that the Oxford and AstraZeneca vaccine can be up to 90% effective if a second dose is administered.
The results may be considered both a triumph and a slight disappointment, following very successful trials by Pfizer and Moderna which showed 95% protection.
The Oxford jab may prove to be crucial for treating people in far flung corners of the world where refrigeration is more difficult.
It is far cheaper and easier to store than other successful vaccines, meaning it will likely play a big part in fighting the pandemic globally if approved.
Health Secretary Matt Hancock said it was “really encouraging news” and he was “really pleased”.
He said the Medicines and Healthcare products Regulatory Agency (MHRA) would still need to pass the Oxford vaccine through safety tests before it could be used, but told Sky News: “We’ve got 100million doses on order and should all that go well the bulk of the rollout will be in the new year”.
Mr Hancock said vaccinations could begin “next month” if they get approval, but it was not clear whether he was talking about the Oxford vaccine specifically in this.
He told BBC Breakfast that if a vaccine is approved by the MHRA: “We hope to be able to start vaccinating next month.
“The bulk of the vaccine rollout programme will be in January, February, March.
“We hope that some time after Easter things will be able to start to get back to normal.”
Oxford University said that interim analysis from its phase three vaccine trial shows that the 70% effectiveness comes from combining two doses.
When volunteers were given two “high” doses the protection was 62%.
This rose to 90% when people were given a “low” dose followed by a high one.
In a statement, Professor Andrew Pollard, director of the Oxford Vaccine Group and chief investigator of the Oxford Vaccine Trial, said: “These findings show that we have an effective vaccine that will save many lives.
“Excitingly, we’ve found that one of our dosing regimens may be around 90% effective and if this dosing regime is used, more people could be vaccinated with planned vaccine supply.
“Today’s announcement is only possible thanks to the many volunteers in our trial, and the hard working and talented team of researchers based around the world.”
Professor Pollard said that no-one who had received the Oxford vaccine in the trials had required hospital treatment for Covid-19.
“We are really pleased with these results,” he told the BBC Radio 4 Today programme.
“What we have got is a vaccine that is able to protect against coronavirus disease and, importantly, there were no hospitalisations or severe cases in anyone who had the Oxford vaccine.
“So, that means that if we did have people vaccinated then certainly so far the results imply that we would be able to stop people getting severe disease and going into hospital.”
The UK has placed orders for 100 million doses of the Oxford vaccine – enough to vaccinate most of the population – with rollout expected in the coming weeks if the jab is approved.
Sarah Gilbert, professor of vaccinology at the University of Oxford, said: “The announcement today takes us another step closer to the time when we can use vaccines to bring an end to the devastation caused by (Covid-19).
“We will continue to work to provide the detailed information to regulators. It has been a privilege to be part of this multi-national effort which will reap benefits for the whole world.”
The UK also has orders for 40 million doses of a jab from Pfizer and BioNTech, which has been shown to be 95% effective.
Another jab from Moderna is 95% effective, according to trial data.
Business Secretary Alok Sharma said the results of an interim analysis of the University of Oxford and AstraZeneca’s coronavirus vacccine candidate were “very promising”.
He tweeted: “Very promising data from the Oxford/AstraZeneca Phase III clinical trials. We are on the cusp of a huge scientific breakthrough that could protect millions of lives.
“The UK has secured early access to 100m doses of their vaccine – on top of 255m doses from other developers.”
The Oxford trials involved more than 20,000 volunteers, half in the UK, the rest in Brazil.
There were 30 cases of Covid in people who had two doses of the vaccine and 101 cases in people who received a dummy injection.
Scientists welcomed the news from the Oxford/AstraZeneca Covid vaccine trial.
Peter Horby, professor of emerging infectious diseases and global health at the University of Oxford tweeted: “Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two.”
Dr Michael Tildesley, associate professor in infectious disease modelling at the University of Warwick, told Times Radio: “It’s absolutely excellent news about the Oxford vaccine because this is really the vaccine that the Government has pinned a lot of their hopes on in terms of resources – we’ve ordered 100 million doses of the Oxford vaccine.”
Work has been going on behind the scenes to ensure that NHS staff are ready to start delivering jabs to the most vulnerable, as well as health and care workers, as a priority.
As positive trial results roll in, vaccination clinics have been proposed, with a number of venues suggested, including sports halls, leisure centres and even the Copper Box stadium in London’s Olympic Park.
The NHS Nightingale Hospitals have also been earmarked as sites for mass vaccination clinics – among other uses.
In addition, NHS leaders have said there will be “roving teams” deployed to vaccinate care home residents and workers.
A trial volunteer said it was a “great feeling” to be involved in the development process.
Sarah Hurst, 47, took part in the AstraZeneca and Oxford University research, receiving two jabs of either the experimental vaccine or a placebo.
She said there was a “tiny sense of pride” at her involvement, but paid tribute to the scientists and researchers who developed the vaccine.