The UK has launched its biggest mass-vaccination programme, aimed at protecting tens of millions of people from Covid-19 within months.
In a race against a faster-spreading variant of coronavirus, ministers have pinned their hopes of ending a third national lockdown on protecting the most vulnerable groups by spring.
But there are huge challenges, not least the unprecedented scale and supply demands but also the need for rigorous safety checks and deep-freeze storage as well as establishing enough vaccination centres and recruiting enough vaccinators.
What is the plan?
The government aims to offer vaccines to 15 million people - the over-70s, healthcare workers and those required to shield - by mid-February and millions more of the over-50s and other priority groups by spring.
They are thought to represent 90-99% of those at risk of dying from Covid-19.
By autumn, the rest of the adult population, another 21 million people, will be offered a vaccine - possibly prioritising front-line workers, such as the police, the fire service and teachers.
The speedy rollout of the vaccine to vulnerable people is seen as critical to reducing the pandemic's death toll and relieving pressure on the NHS.
But to meet its February target, ministers need to deliver more than two million jabs a week by the end of January, in one of the largest civilian logistical operations launched in in the UK.
Prime Minister Boris Johnson has said it will require an "unprecedented national effort" but the government is throwing "everything at it" to deliver "hundreds of thousands" of jabs each day.
Almost five million people had now received the first of two doses of a vaccine, Health Secretary Matt Hancock said on Thursday.
The NHS began administering a vaccine made by Pfizer-BioNTech at the beginning of December. But the operation has been significantly ramped up following the approval of a second vaccine, from Oxford University and AstraZeneca.
A third vaccine, from Moderna, has also been approved.
The campaign to reach as many people as possible as quickly as possible was boosted by a shift in policy in early January - to prioritise the first dose of either vaccine, with a second dose up to 12 weeks later, a bigger gap than originally planned.
How many have been vaccinated so far?
The government needs to administer about 380,000 vaccine doses each day through the UK's National Health Service (NHS) if it is to reach the 15 million most vulnerable people by the middle of February.
While the current seven-day average achieved has been almost 307,000, on Thursday more than 400,000 first doses were administered for the first time. If this is maintained, the target will be reached.
Overall, more than five million people across the UK have now received their first dose.
Mr Hancock has also said 63% of residents in elderly care homes have now received a vaccine.
This progress means the UK continues to be among the countries with the highest vaccination rates globally.
But within the UK, there is some regional variation.
North East and North West England have reached the highest percentage of over-80s with their first doses so far.
Within England, eight areas had given the first dose of the vaccine to fewer than 50% of the over 80s by 17 January.
Suffolk and North East Essex currently has the lowest proportion at 36%. But there are four areas where that figure is more than 75%, with Gloucestershire at 85%.
The health secretary has told MPs that supply of the vaccine must be fairly distributed across the country, with everyone in the top four priority groups receiving an offer of a vaccine by 15 February.
Where are the vaccines coming from?
The UK is currently receiving doses of two vaccines approved by the medicine regulator. The Pfizer-BioNTech jab - the first given the green light - is being imported from Puurs, Belgium.
The Oxford vaccine, meanwhile, is being made in Britain, by two biotech companies:
- Oxford BioMedica, based in Oxford
- Cobra Biologics, based at Keele Science Park, Staffs
Another company, Wockhardt, based in Wrexham, fills the vials and packages them for use.
Supplies of the third vaccine to be approved, made by US company Moderna, will come from Switzerland or Spain but are not expected to be available until spring.
Are there hold-ups?
There are a number of challenges in what is called the vaccine "supply chain" - the logistics of how the jab gets from manufacturers to people.
Getting enough supplies in the first place, checking those supplies are up to scratch and transporting vaccines according to their requirements have all thrown up difficulties.
The health secretary told the House of Commons on Thursday supply was "lumpy", but manufacturers were working "as fast as possible".
The number of doses of the Pfizer vaccine will be lower than expected this month because the company is upgrading its factory in Belgium in order to increase production in March.
The prime minister has also referred to the "rate-limiting factor" of batch testing - the process of ensuring vaccines released by manufacturers are safe and up to standard.
The UK's Medicines and Healthcare Products Regulatory Authority (MHRA) requires vaccines to be checked by the National Institute of Biological Standards and Control (NIBSC) to ensure they are effective, structurally intact and free of contaminants.
This process can take a long time as it has to be done twice - before and after the vaccine enters vials. Ensuring a batch is sterile can take two weeks.
To deal with the challenge, the NIBSC had scaled up its capacity so "multiple batches can be tested simultaneously" and more technical staff are being taken on.
There are also challenges when transporting the vaccines.
While the Oxford vaccine can be stored in fridges and transported in regular refrigerated vans or cool boxes, the Pfizer jab - made from genetic material - needs to be stored at -70C to prevent it from degrading. This means it needs to be transported in a carefully controlled deep-freeze delivery chain.
How will people be vaccinated?
People will be vaccinated in three main ways, at:
- local GP practices and community pharmacies
- hospital hubs
- mass vaccination sites across the country
The government has urged the public to "play their part" in supporting "the largest vaccination programme in British history", including helping people attend their appointments.
In England, about 1,000 GP sites, 200 community pharmacies, 206 hospital hubs and 17 mass vaccination sites are offering services.
In Wales, the vaccine is being distributed at GP practices and by mobile units. And they hope to have 35 mass vaccination centres up and running in the coming weeks.
In Scotland, as well as GPs surgeries, pharmacies and hospital hubs, there will also be a number of larger vaccination sites.
In Northern Ireland, vaccinations are taking place at GP practices and community halls and being delivered by mobile teams to care homes. Health and social care workers are also being vaccinated at seven main vaccination sites. These sites could be used to vaccinate members of the public as the programme expands later in the year.
Thousands of people have been trained to deliver the vaccines, with thousands more set to join the effort. The charity St John Ambulance Brigade is among those helping out.
And a further 21 quick-reaction vaccination teams will also be ready to deployed anywhere around the country, commander of military support to the vaccine delivery programme Brig Phil Prosser says.
The prime minister says no-one should have to travel more than 10 miles for a jab.
Is there enough vaccine?
The UK has ordered 100 million doses of the Oxford vaccine - enough to inoculate 50 million people.
This, when combined with the 40 million ordered Pfizer jabs, will cover the entire population, the health secretary has said.
The UK also has significant orders of the newly-approved Moderna vaccine and four other candidates.
But having vaccines on order is not the same as having them ready to go. Of the 100 million Oxford jabs ordered, only 530,000 were ready for nationwide rollout on 4 January. Although, the government has said this number will rise to "tens of millions" by the end of March.
Pfizer says the number of doses it has sent to the UK is now "in the millions".
The company has said that although shipments to the UK would be affected by upgrades to its production process this month, overall the country would receive the agreed volumes for the first three months of the year.
Design by Lilly Huynh, Irene de la Torre Arenas and Sana Jasemi. Additional reporting by Smitha Mundasad