Hopes have been raised that vaccines could end the coronavirus pandemic after a British jab was found to be up to 90% effective in preventing Covid-19.
AstraZeneca and Oxford University said their jab is effective in stopping most people from contracting coronavirus and falling seriously ill, with some indications that it can also prevent people passing the virus to others.
The jab is likely to be rolled out in the UK from December, with the bulk of vaccination in the new year.
Tom Keith-Roach, president of AstraZeneca UK, told the PA news agency the UK can expect to receive 19 million doses of the jab by the end of this year.
One of the dosing patterns used by the scientists - and tested on about a third of those in the study - suggested 90% effectiveness if one half dose is given followed by a further full dose.
Another dosing pattern showed 62% efficacy when one full dose is given followed by another full dose.
The combined analysis from both dosing regimes resulted in an average efficacy of 70.4%, better than the average flu vaccine.
Experts behind the study believe that a smaller initial dose may prime the immune system to give a bigger, better response when it meets coronavirus.
The half dose regime is also thought to prevent transmission of the virus and experts hope regulators will approve this plan.
During the overall clinical trial, those given the vaccine did not suffer severe coronavirus and nobody required hospital treatment, while there were also no serious safety concerns related to the vaccine itself.
Health Secretary Matt Hancock said the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK will now assess if the 90% effectiveness dosing regime can be used.
He said: "I'm really very pleased, I really welcome these figures - this data that shows that the vaccine in the right dosage can be up to 90% effective.
"If this all goes well in the next couple of weeks, then we are looking at the potential of starting the vaccination programme next month for this Oxford-AstraZeneca vaccine as well as the Pfizer vaccine.
"But in all cases the bulk of the rollout will be in the new year.
"We are looking with high confidence now that after Easter things can really start to get back to normal."
Prime Minister Boris Johnson tweeted: "Incredibly exciting news the Oxford vaccine has proved so effective in trials.
"There are still further safety checks ahead, but these are fantastic results."
Professor Andrew Pollard, director of the Oxford Vaccine Group, said it was "a very exciting day".
"These findings show that we have an effective vaccine that will save many lives.
"We have a vaccine for the world because we've got a vaccine which is highly effective - it prevents severe disease and hospitalisation.
"I think this is an incredibly exciting moment for human health."
Prof Pollard told reporters the 90% effectiveness finding has already met the "necessary statistical evidence as required by regulators".
He said further evidence will probably be available next month but it is "a highly significant result even with the numbers that we have".
He also celebrated the suggestion that the vaccine could reduce asymptomatic infection.
"If that is right, we might be able to halt the virus in its tracks and stop transmitting between people," he said.
AstraZeneca plans to have 700 million doses of the jab globally by the end of March, with 40 million doses in the UK by the end of March, according to the firm.
"We have a robust supply chain which is capable of manufacturing at scale and we can do it very quickly," Pam Cheng, executive vice president of operations and information technology at AstraZeneca, told reporters.
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)."
She said scientists are "optimistic" that the Oxford Covid-19 vaccine will offer durable protection, with other trials using the same technology showing a strong immune response maintained a year after vaccination.
On the trial results suggesting that a half dose could prime the immune system, she said more work is needed.
But she added: "It could be that by giving a small amount of the vaccine to start with and following up with a big amount, that's a better way of kicking the immune system into action and giving us the strongest immune response."
Prof Gilbert said experts working on vaccines globally are not "thinking about vaccinations working in terms of one person at a time".
She added: "We have to think about vaccinating communities, populations, reducing transmission within those populations, so that we really get on top of this pandemic.
"That's what it now looks like we're going to have the ability to contribute to in a really big way."
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.
It also has orders for 40 million doses of the jab from Pfizer and BioNTech, which has been shown to be 95% effective.
Another jab from Moderna, of which the UK has five million doses on order, is 95% effective, according to trial data.
The FTSE 100 leading index of shares was up 0.35% in the first hour of trading following the news, with growth coming mainly from companies hoping for travel to start up again.
British Airways owner IAG, engine-maker Rolls-Royce, and oil giants Shell and BP were all early risers on the stock market.
Online learning group Pearson and vaccine maker AstraZeneca were the biggest fallers - with traders seemingly less hopeful of making as much cash from the vaccine as previously thought.
Peter Horby, professor of emerging infectious diseases and global health at the University of Oxford, welcomed the fact that the vaccine can be stored in a fridge rather than the minus 70C to minus 80C needed for the Pfizer and Moderna vaccines.
He tweeted: "Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two."
In a statement, he added: "This is very welcome news, we can clearly see the end of tunnel now."
Professor Azra Ghani, chair in infectious disease epidemiology at Imperial College London, said: "A particular strength of this vaccine is that it can be stored in a fridge; this means that it can be distributed around the world using existing delivery mechanisms.
"This could therefore have a truly significant impact across the globe and enable an end to the Covid-19 pandemic."