The UK approved the Pfizer-BioNTech vaccine for use and the shot will be available from next week.
Britain is the first country in the West to approve a coronavirus vaccine for general use in an attempt to end the pandemic that began in Wuhan in China late last year and has so far killed more than 1.4 million people.
The approval came on the same day England emerged from its second lockdown.
The vaccine was approved by the Medicines and Healthcare products Regulatory Agency (MHRA).
Immunisations could start within days, with the shot made available across the UK from next week, the government said.
Prime Minister Boris Johnson tweeted that the rollout of vaccines would "ultimately allow us to reclaim our lives and get the economy moving again".
It’s fantastic that @MHRAgovuk has formally authorised the @Pfizer/@BioNTech_Group vaccine for Covid-19. The vaccine will begin to be made available across the UK from next week. (1/2)
— Boris Johnson (@BorisJohnson) December 2, 2020
It’s the protection of vaccines that will ultimately allow us to reclaim our lives and get the economy moving again. (2/2)
— Boris Johnson (@BorisJohnson) December 2, 2020
Health Secretary Matt Hancock said "help is on its way" and confirmed hospitals would begin a mass vaccination programme for priority groups from "early next week".
"The NHS (National Health Service) stands ready to make that happen," he told Sky News.
"From early next week, we will start the programme for vaccinating people against Covid-19 in this country."
Pfizer and BioNTech on Tuesday sought regulatory clearance for their vaccine in the European Union, putting the shot on track for potential approval on the continent before the end of the year.
In the US, a Food and Drug Administration panel is to meet on December 10 to discuss the vaccine.
Covid-19 vaccines: everything you need to know
Britain was able to approve the vaccine ahead of Europe under special regulations put in place before Brexit on January 1, when the UK becomes fully responsible for medicines authorisation.
Trials showed Pfizer's vaccine was 95 per cent effective in protecting people from coronavirus.
The UK has ordered 40 million doses of the vaccine – enough to vaccinate 20 million people because it is a two-shot regimen.
The government said the health service had made "extensive preparations" for a mass immunisation programme that will begin with care home residents, health workers, the elderly and the extremely clinically vulnerable as first in line to be inoculated.
But it said coronavirus restrictions must still be adhered to until most of the population is inoculated.
About 10 million doses should be available soon, with the first 800,000 arriving in the UK from Belgium in the coming days.
The shot will be administered by GPs, pharmacists and at vaccination centres which are being set up around the country.
While care home residents are first in line to get the shot, this depends on whether it is practically possible.
Adding to the distribution challenges, the Pfizer vaccine must be stored at ultra-cold temperatures. Two doses three weeks apart are required for protection.
The vast majority of vaccinations are expected next year.
Mr Hancock predicted the UK would begin to return to normality in the first half of 2021.
"From the spring onwards, things will start to get better," he told BBC's Radio 4 Today programme.
Experts hailed Britain's emergency use authorisation as a historic moment in the global fight against coronavirus and a triumph for science.
It is the fastest vaccine to go from concept to reality, taking only 10 months to follow the same steps that normally span a decade.
Danny Altmann, professor of immunology at Imperial College London, said news of the approval was momentous.
He said: "Less than 11 months from the first characterisation of the virus sequence, we have the first emergency approval for use of a really effective vaccine. Truly heroic.
"I don't think we should get too hung up on ‘the race’ and this as the first approval. Over the next several weeks we’ll probably see a number of licences granted – and we do need them all to get speedily out of this mess."
Liam Smeeth, professor of clinical epidemiology at the London School of Hygiene and Tropical Medicine, said the UK was charting a course towards a "much better situation".
"A further circuit breaker in January or possibly February is likely to be needed. But it is realistic to hope that by March or April the vast majority of older people, care home residents, and those with severe conditions will have been immunised," he said.
"Life won’t ever be the same as it was before Covid-19, but it will feel a whole lot better than now."
How does Pfizer's vaccine work?
Pfizer's 95 per cent effective shot is an mRNA vaccine.
Traditional vaccines inject people with a dead or weakened part of a virus so the body produces antibodies to fight it, as it would in a natural infection.
But mRNA vaccines differ in that they encourage the body to become its own miniature vaccine factory.
The vaccine, an abbreviation of "messenger RNA", delivers genetic instructions that prompt the body to produce virus proteins – without exposing the body to any threat.
Once this happens, the immune system begins to build up protective antibodies to guard against infection.
What are the key differences between the vaccines?
The Pfizer vaccine needs to be kept at minus 70°C while in storage, which may present headaches for health officials who have to source ultra-cold freezers.
The Moderna vaccine can to be stored for up to six months at minus 20°C.
While the AstraZeneca/Oxford vaccine has lower efficacy than the Pfizer and Moderna shots, it is cheaper, easier to store and and therefore more practical to distribute to the world than the two rivals.
The Oxford dose has an average 70 per cent effectiveness rate.
The trials showed a regimen of two full doses one month apart was 62 per cent effective, but a half dose followed by another full shot showed 90 per cent.
