The coronavirus vaccine developed by the University of Oxford is highly effective at stopping people developing Covid-19 symptoms, a large trial shows.
Overall results showed 70% protection, but the researchers say the figure may be as high as 90% by tweaking the dose.
The results will be seen as a triumph, but also come off the back of Pfizer and Moderna showing 95% protection.
However, the Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two.
So the vaccine will play a significant role in tackling the pandemic, if it is approved for use by regulators.
"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 [the virus]," said the vaccine's architect Prof Sarah Gilbert.
The UK government has pre-ordered 100 million doses of the Oxford vaccine and AstraZeneca says it will make three billion doses for the world next year.
Prime Minister Boris Johnson said it was "incredibly exciting news" and that while there were still safety checks to come, "these are fantastic results".
The vaccine has been developed in around 10 months, a process that normally takes a decade.
What did the trial show?
More than 20,000 volunteers were involved, 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.
The researchers said it works out at 70% protection.
When volunteers were given two "high" doses the protection was 62%, but this rose to 90% when people were given a "low" dose followed by a high one. It's not clear why there is a difference.
"We're really pleased with these results," Prof Andrew Pollard, the trial's lead investigator, told the BBC.
He said the 90% effectiveness data was "intriguing" and would mean "we would have a lot more doses to distribute."
There were also lower levels of asymptomatic infection in the low-followed-by-high-dose group which "means we might be able to halt the virus in its tracks," Prof Pollard said.
When will I get it?
In the UK there are four million doses ready to go, with another 96 million to be delivered.
But nothing can happen until the vaccine has been approved by regulators who will assess the vaccine's safety, effectiveness, and that it is manufactured to high standard. This process will happen in the coming weeks.
However, the UK is ready to press the go button on an unprecedented mass immunisation campaign that dwarfs either the annual flu or childhood vaccination programmes.
Care home residents and staff will be first in the queue, followed by healthcare workers and the over-80s. The plan is to then to work down through the age groups.
How does it work?
The vaccine is a genetically modified common cold virus that used to infect chimpanzees.
It has been altered to stop it causing an infection in people and to carry the blueprints for part of the coronavirus, known as the spike protein.
Once these blueprints are inside the body they start the producing the coronavirus' spike protein, which the immune system recognizes as a threat and tries to squash it.
When the immune system comes into contact with the virus for real, it now knows what to do.
Are the results disappointing?
After Pfizer and Moderna both produced vaccines delivering 95% protection from Covid-19, a figure of 70% will be seen by some as relatively disappointing.
However, anything above 50% would have been considered a triumph just a month ago and 70% is comfortably better than the seasonal flu jab.
This is still a vaccine that can save lives from Covid-19.
It also has crucial advantages that make it easier to use. It can be stored at fridge temperature, which means it can be distributed to every corner of the world, unlike the Pfizer/BioNTech and Moderna vaccines, which need to be stored at much colder temperatures.
Oxford's manufacturing partner, AstraZeneca, is preparing to make three billion doses worldwide.
The Oxford vaccine, at a price of around £3, also costs far less than Pfizer's (around £15) or Moderna's (£25) vaccines.
What difference will this make to my life?
A vaccine is what we've spent the year waiting for and what lockdowns have bought time for.
However, producing enough vaccine and then immunising tens of millions of people in the UK, and billions around the world, is still a gargantuan challenge.
Life will not return to normal tomorrow, but the situation could improve dramatically as those most at risk are protected.
Health Secretary Matt Hancock told BBC Breakfast we would be "something closer to normal" by the summer but "until we can get that vaccine rolled out, we all need to look after each other".
What's the reaction been?
Prof Peter Horby, from the University of Oxford by not involved in the trial, said: "This is very welcome news, we can clearly see the end of tunnel now. There were no Covid hospitalisations or deaths in people who got the Oxford vaccine."
Dr Stephen Griffin, from the University of Leeds, said: "This is yet more excellent news and should be considered tremendously exciting. It has great potential to be delivered across the globe, achieving huge public health benefits.