There are already two other vaccines – why is this a big deal?
Such strong results from the Oxford-AstraZeneca team give the world real hope of an end to the pandemic early next year.
While the Pfizer and Moderna jabs will play a vital role, the Oxford vaccine is likely much better suited to mass immunization involving billions of people.
The Oxford results also suggest that the vaccination may stop someone becoming infected and passing it on.
The previous results only showed a jab would drastically cut the odds of someone getting ill.
Why is it easier to roll out?
The Oxford vaccine can be stored and transported in a normal fridge, while the other two require freezing.
Such strong results from the Oxford-AstraZeneca team give the world real hope of an end to the pandemic early next year
The jab is also much cheaper, having reached agreements with governments and international health organizations to sell it at cost price for around $2.50 a dose.
Pfizer's vaccine costs about $20, while Moderna's is $15 to $25, based on agreements the companies have struck to supply their vaccines to the US government.
All three vaccines must be approved by regulators before they can be widely distributed.
The Oxford jab is also based on well-established technology, meaning it is easier to manufacture at large scale.
What does it mean for the US?
Before the vaccine is rolled out in the U.S., it has to be approved by the Food and Drug Administration.
The FDA initially set the bar for a Covid vaccine at 50 percent effectiveness, the same as the seasonal flu vaccine.
The US has reportedly agreed to procure 500 million doses of the Oxford-AstraZeneca vaccine, and many other countries have deals in place.
The government had already agreed to buy 100 million doses of the Pfizer vaccine at $19.50 a shot, and an option is in place to buy a further 500 million.
Pfizer hopes to produce up to 50 million doses by the end of the year.
With the Moderna vaccine, the US contributed $955 million to its development and will get first access to 100 million doses for $1,525 billion.
AstraZeneca has been ramping up manufacturing capacity, so it can supply hundreds of millions of doses of the vaccine starting in January.
Scientists are not entirely sure, but believe a half dose 'primes' the immune system and the second full dose boosts that protection
So which is the more effective vaccine?
The data is roughly the same, with each vaccine giving roughly 90 percent protection at the optimal dose.
Pfizer's final data showed its shot is about 95 percent effective, Moderna's blocked 94.5 percent of infections and initial findings show Oxford's is up to 90 percent effective, and 70 percent effective on average.
Why is an initial half dose more effective?
Scientists are not entirely sure, but believe a half dose 'primes' the immune system and the second full dose boosts that protection.
Giving a full dose for the first jab may lead to the immune system over-reacting and killing off the vital cells on which the vaccine relies.
How soon will we get the jabs?
The first people are likely to start receiving both the Oxford and Pfizer vaccines next month although full data is not yet available for the Oxford jab.
As soon as the formal decision is made by the FDA, Operation Warp Speed aims to distribute shots within 24 hours of the emergency approval. Officials say that injections will be able to start another 24 hours after that.
The vaccine advisory committee is set to meet on December 10 to discuss authorization of the Pfizer vaccine.
Dr Moncef Slaoui, chief scientific adviser for Operation Warp Speed, said it could then be rolled out within 24 hours of approval.
Moderna is expected to seek separate approval later in December for its vaccine.
Who will get it first?
Individual states will be responsible for deciding who receives the vaccine first.
It is recommended that those most at risk should be vaccinated first.
Most US experts say healthcare workers should be the very first line.
An advisory panel that will guide the CDC's plan said health care workers should remain first priority while supplies are limited.
Next will likely be other essential workers, like grocery and delivery staff, elderly people and those with chronic diseases like heart disease and diabetes that make COVID-19 deadlier.
Vaccines will probably be available to the general American public beginning in the spring or early summer.
It is recommended that those most at risk including the elderly and healthcare workers are first in line to receive a vaccine
So which vaccine will I get?
The Pfizer vaccine is the most likely to be rolled out first but tests may be carried out to determine whether different jabs work better for different groups of people.
It is possible that the Oxford vaccine will be used more in local vaccination centers because it is easily stored.
That could be key for rural, poor and tribal areas of the U.S. which are being hard-hit by the winter surge of the virus.
The Pfizer and Moderna jabs could be used in the mass vaccination centers including drive-thru facilities and sports stadiums, which will have space to set up specialist freezers.
How soon until I am safe?
It takes six weeks for immunity to kick in from the first jab. There is a gap of four weeks between the two doses, then two more weeks until protection starts.
And how long will I be protected for?
That is not yet clear, but the Oxford team believes their jab should give at least a year's protection, suggesting everyone will need to have an annual vaccine.
The Oxford jab is likely to be used in pop-up vaccination centers, since it can be stored in fridges
What are the risks?
The full data has not yet been published, but each shot certainly looks safe. The Oxford jab has been used on volunteers since April – with no major safety concerns – and the other two have at least two months of safety data.
Side effects discussed by the jab-makers and trial participants may include aching and fatigue that some have compared to the flu, or hangovers, but the unpleasant effects subsided within a day or two, and many participants did not experience them.
Altogether more than 100,000 people have been involved in the trials which have produced data. Usually a vaccine will receive a license after trials on 2,000 to 3,000 people.
Will it mean an end to Covid?
Experts are increasingly confident that, at the very least, vaccination will spell an end to Covid restrictions sometime next year.
Scientists have previously predicted a vaccine that gives 80 percent protection will mean no need for social distancing – a result of more than 90 percent is well in excess of that.
It might take some years for the world's population to be vaccinated, and Covid may spring back from time to time, but an effective vaccine means lockdowns will eventually become obsolete.
Dr Anthony Fauci has predicted that vaccines could return the U.S. to some semblance of normality by fall 2021.