The UK didn't cut any corners when it approved Pfizer's rapidly-developed coronavirus vaccine, the head of the medicines regulator says.
Dr June Raine, head of the Medicines and Healthcare products Regulatory Agency (MHRA), assured the public that the world's first approved Covid-19 jab will be safe.
Britain is the first country to give emergency use approval to the Pfizer and BioNTech vaccine which was developed in just a matter of months.
The first jabs will be administered next week and priority will be given to care home residents and staff, the elderly and people in at-risk groups, and NHS staff working on the frontline of the pandemic.
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Health Secretary Matt Hancock predicted the UK would be through Covid "by spring", and he urged Britons to keep their guards up and continue to follow social distancing and mask rules.
Speaking to reporters, Dr Raine said "no corners have been cut" in assessing the vaccine's safety.
The MHRA chief told a Downing Street briefing: "The safety of the public will always come first.
"This recommendation has only been given by the MHRA following the most rigorous scientific assessment of every piece of data so that it meets the required strict standards of safety, of effectiveness and of quality."
The jab has been shown in studies to be 95% effective and works in all age groups.
The UK has ordered 40 million doses of the Pfizer vaccine, enough to vaccinate 20 million people - or about a third of the population - with two doses, given 21 days apart.
Health Secretary Matt Hancock said 800,000 doses of the jab will arrive from a Pfizer plant in Belgium next week, with millions more doses in the coming weeks.
The Joint Committee on Vaccination and Immunisation (JCVI), which advises ministers, said vaccines should first be offered to elderly people in care homes and care home workers.
Next on the priority list are those aged 80 and above and frontline health workers.
All those aged 75 and over should be vaccinated next, followed by those 70 and over and clinically "extremely vulnerable" individuals, it said.
People aged 65 and over are next in line, alongside anyone aged 16 to 64 who has underlying health conditions which put them at a "higher risk of serious disease and mortality".
Those aged 60 and over will be vaccinated next, followed by those aged 55 and over, and then those aged 50 and over.
No decisions have yet been made on priorities for under-50s.
Studies on the vaccine show it has to be stored at minus 70C but is also stable at 2C to 8C for a short time, meaning it could possibly be sent to different locations.
But it is thought operational reasons may prevent care home residents getting the jab first. The Department of Health has been contacted for comment.
Prime Minister Boris Johnson said the announcement was "fantastic" news, tweeting: "It's the protection of vaccines that will ultimately allow us to reclaim our lives and get the economy moving again."
A Department of Health and Social Care spokesman said the Medicines and Healthcare products Regulatory Agency (MHRA) had approved the jab after "months of rigorous clinical trials and a thorough analysis of the data by experts" from the regulator.
He said they have concluded that the vaccine has "met its strict standards of safety, quality and effectiveness".
Mr Hancock told Sky News the UK was the first country in the world to have a "clinically authorised vaccine" and it would be deployed as "quickly as it is manufactured".
He added: "This is fantastic news.
"The MHRA, the fiercely independent regulator, has clinically authorised the vaccine for roll out.
"The NHS stands ready to make that happen.
"So, from early next week we will start the programme of vaccinating people against Covid-19 here in this country."
Mr Hancock said until vaccines were rolled out people needed to stick to the rules, saying: "We've got to get from here to there and we've got to keep people safe in the meantime."
There would be "three modes of delivery" of the vaccine, with hospitals, mass vaccination centres and GPs and pharmacists offering the jab to those most in need, he added.
"Fifty hospitals across the country are already set up and waiting to receive the vaccine as soon as it's approved, so that can now happen."
Mr Hancock confirmed vaccinations will start with the most elderly, people in care homes and their carers, with NHS staff and the clinically extremely vulnerable also high on the list.
He told BBC Breakfast: "2020 has been just awful and 2021 is going to be better, and help is on its way with this vaccine.
"I'm confident now with the news today that from spring, from Easter onwards, things are going to be better and we're going to have a summer next year that everybody can enjoy."
England's chief medical officer, Professor Chris Whitty, said the vaccine approval is "excellent news and a step towards normality".
He tweeted: "It will take until spring until the vulnerable population who wish to are fully vaccinated. We can't lower our guard yet."
Nadhim Zahawi, the newly-appointed minister responsible for overseeing the vaccination roll-out, tweeted: "Major step forward in the fight against Covid-19 today."
In a Downing Street briefing, Professor Wei Shen Lim from the JCVI explained how care home residents were vulnerable to Covid-19, hence why they are top of the list.
But he added: "Whether or not the vaccine itself can be delivered to care homes is obviously an important point, and there will be some flexibility in terms of operational constraints.
"The JCVI's advice is that every effort should be made to supply vaccines and offer vaccinations to care home residents, whether or not that is actually doable is dependent on deployment and implementation."
Professor Sir Munir Pirmohamed, chair of the Commission on Human Medicine expert working group, added: "We did obviously look at the stability of the vaccine, as you said it is stored at -70 degrees.
"But we were able to look at stability data and there's stability data showing that it is stable for a short period of time at two to eight degrees which allows it to be transported to the relevant vaccination sites."
Dr June Raine, head of the MHRA, said "no corners have been cut" in assessing its safety.
"The safety of the public will always come first," she said. "This recommendation has only been given by the MHRA following the most rigorous scientific assessment of every piece of data so that it meets the required strict standards of safety, of effectiveness and of quality."
Information obtained by the PA news agency on the jab's roll-out says once the vaccine arrives in the UK from Pfizer's plant in Belgium, batches will be checked at a central depot to ensure their quality.
The vaccine will then be unloaded and moved to storage freezers where it will undergo an additional temperature check.
Public Health England (PHE) will process orders placed by the NHS for next day delivery to hospital hubs around the UK.
Defrosting the vaccine for use takes several hours and then extra time is needed to prepare the vaccine for administering as doses.
Ben Osborn, Pfizer's UK country manager, told PA deliveries were already under way from the Pfizer plant to the UK but said he was not "giving an absolute figure" on how many doses would be delivered to the UK this year.
Scotland's First Minister Nicola Sturgeon said the approval of the vaccine was "the best news in a long time".
Sir Simon Stevens, chief executive of the NHS in England, said the vaccination programme would be the "largest scale vaccination campaign in our country's history".
Business Secretary Alok Sharma tweeted: "In years to come, we will remember this moment as the day the UK led humanity's charge against this disease."
