Britain is the first country to approve Pfizer's coronavirus vaccine for emergency use and Health Secretary Matt Hancock says the UK will be through Covid-19 "by spring".
A mass vaccination campaign will begin next week when the first 800,000 doses arrive from Pfizer's plant in Belgium, with the bulk of the rollout taking place in the new year.
The UK has ordered enough doses of the two-dose vaccine to inoculate 20million people, or about a third of the population.
The Pfizer-BioNTech jab has been shown in studies to be 95% effective and works in all age groups, and it has been developed at incredible speed.
A process which usually takes years has been condensed to months.
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A vaccine is seen as the best chance for the world to get back to some semblance of normality amid a global pandemic which has killed nearly 1.5million people and upended the global economy.
This is how the world's first approved Covid-19 vaccine works and why it was approved so quickly.
What type of vaccine is this?
The jab is known as a messenger RNA (mRNA) vaccine.
Conventional vaccines are produced using weakened forms of the virus, but mRNAs use only the virus's genetic code.
An mRNA vaccine is injected into the body where it enters cells and tells them to create antigens.
These antigens are recognised by the immune system and prepare it to fight coronavirus.
What are the advantages of this type of vaccine?
No actual virus is needed to create an mRNA vaccine. This means the rate at which it can be produced is dramatically accelerated.
As a result, mRNA vaccines have been hailed as potentially offering a rapid solution to new outbreaks of infectious diseases.
In theory, they can also be modified reasonably quickly if, for example, a virus develops mutations and begins to change.
mRNA vaccines are also cheaper to produce than traditional vaccines, although both will play an important role in tackling Covid-19.
One downside to mRNA vaccines is that they need to be stored at ultra-cold temperatures and cannot be transported easily.
Are they safe?
All vaccines undergo rigorous testing and have oversight from experienced regulators.
Some believe mRNA vaccines are safer for the patient as they do not rely on any element of the virus being injected into the body.
mRNA vaccines have been tried and tested in the lab and on animals before moving to human studies.
The human trials of mRNA vaccines - involving tens of thousands of people worldwide - have been going on since early 2020 to show whether they are safe and effective.
Pfizer will continue to collect safety and long-term outcomes data from participants for two years.
Why was it approved so quickly?
Analysis shows the Pfizer vaccine can prevent 95% of people from getting Covid-19, including 94% in older age groups.
The vaccine has been tested on 43,500 people in six countries and no safety concerns were raised.
Approval means the UK can begin rolling out the vaccine to those most in need, including frontline NHS workers.
In the past it has taken years, sometimes decades, to produce a vaccine.
Traditionally, vaccine development includes various processes, including design and development stages followed by clinical trials - which in themselves need approval before they even begin.
But in the trials for a Covid-19 vaccine, things look slightly different. A process which usually takes years has been condensed to months.
While the early design and development stages look similar, the clinical trial phases overlap, instead of taking place sequentially.
And pharmaceutical firms have begun manufacturing before final approval has been granted - taking on the risk that they may be forced to scrap their work.
The new way of working means that regulators around the world can start to look at scientific data earlier than they traditionally would do.
Aren't there other vaccines?
Yes, recent data from the Oxford/AstraZeneca, and Moderna vaccine trials suggests their candidates also have high efficacy.
Oxford data indicates the vaccine has 62% efficacy when one full dose is given followed by another full dose, but when people were given a half dose followed by a full dose at least a month later, its efficacy rose to 90%.
The combined analysis from both dosing regimes resulted in an average efficacy of 70.4%.
Final results from the trials of Moderna's vaccine suggest it has 94.1% efficacy, and 100% efficacy against severe Covid-19.
Nobody who was vaccinated with the vaccine known as mRNA-1273 developed severe coronavirus.
Which jab is best?
The early contenders all have high efficacy rates, but researchers say it is difficult to make direct comparisons because it is not yet known exactly what everyone is measuring in the trials.
According to reports, the Moderna vaccine could cost about £28 per dose and the Pfizer candidate could cost around £15.
Researchers suggest the Oxford vaccine could be relatively cheap to produce, with some reports indicating it could be about £3 per dose.
However, the details of the deals made by the UK Government have not been made public.
