Almost 18 million people in the UK have received at least one dose of a coronavirus vaccine - part of the biggest inoculation programme the country has ever launched.

In a race against a faster-spreading variant of the virus, 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.

The UK vaccine programme has entered its second phase after everyone in the first four priority groups - those aged 70 and over, care home residents, healthcare workers and people required to shield - were offered a jab by mid-February.

The rollout is now being expanded to include those aged 60 and above and those deemed clinically vulnerable. The rest of the over-50s will follow, with the government aiming to offer everyone in priority groups five to nine a jab by mid-April.

These groups account for 88% of deaths so far.

More than 17.9 million people so far have had a first vaccine dose - equivalent to one in three adults in the UK. More than 642,000 have had a second.

The number of first doses administered each day had been steadily climbing since December - reaching more than 400,000 a day mid-February.

However, the current seven-day average for first doses is about 334,000 doses a day.

Prof Jonathan Van-Tam, England's deputy chief medical officer, acknowledged there were "supply fluctuations" for the vaccines and said it could take "a few months" for manufacturers to get into a steady routine for production.

The campaign to reach as many people as quickly as possible was boosted by a shift in policy in early January - to prioritise the first dose of a vaccine, with a second dose up to 12 weeks later, a bigger gap than originally planned.

The speedy rollout of the vaccine to all vulnerable people is seen as critical to reducing the pandemic's death toll and relieving pressure on the NHS.

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The progress made in the UK so far means the country continues to be among those with the highest vaccination rates globally.

From spring, the government plans to begin vaccinating the rest of the adult population, another 21 million people.

Within the UK, there has been some regional variation in the vaccination rollout - in particular between the nations.

All English regions except London have now jabbed more than 90% of people aged 70 or above with their first dose, a total of 7.1 million people according to weekly data released on Thursday last week.

A further 452,008 people in the same age range have received their first dose in Wales (90%) and 676,325 people in Scotland (91%) - as of 18 February.

No equivalent figure for Northern Ireland was available, but the Northern Ireland Health Department said 409,507 first doses had been given in total, including 94% of people aged 80 or older (67,824).

There are also disparities between ethnic groups and poorer and wealthier areas.

Analysis of NHS records by the OpenSAFELY group - a collaboration between Oxford University and the London School of Hygiene and Tropical Medicine - shows that black people were the least likely to have received a vaccine among those aged 80 and over, while white people were the most likely.

The study was based on more than 20 million patient records in England and covers people not living in care homes. Areas of London are under-represented in the data.

In addition, 85% of those aged 80 and over in the poorest areas had been given the vaccine by 11 February compared with 94% in the most affluent areas.

The UK is currently receiving doses of two vaccines approved by the medicine regulator.

The Pfizer-BioNTech jab - the first to be given the green light in December - is being imported from Puurs, Belgium.

A second vaccine, from Oxford University and AstraZeneca, is being made in Britain by two biotech companies: Oxford BioMedica, based in Oxford; and Cobra Biologics, based at Keele Science Park, Staffs.

Another company, Wockhardt, based in Wrexham, fills the vials and packages them for use.

Supplies of a third vaccine to be approved, made by US company Moderna, will come from Switzerland or Spain but are not expected to be available until later next month.

The UK is also lined up to receive at least three other vaccines if they are approved for use.

A jab manufactured by US firm Novavax will be made in Stockton-on-Tees in north-east England, while another by French company Valneva will be made in Livingston, West Lothian, Scotland.

The third, by Belgian firm Janssen, owned by Johnson & Johnson, should also be available later this year.

People will be vaccinated in three main ways, at:

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, more than 1,500 sites, including GPs' surgeries, community pharmacies and hospital hubs, are offering vaccination services.

They also include 90 major vaccine sites which have been set up in larger premises, such as sports centres, racecourses and showgrounds, which are capable of giving jabs to thousands of people a week.

In Wales, the vaccine is being distributed at GP practices and by mobile units. And they hope to have 35 mass vaccination centres also up and running.

In Scotland, as well as GPs surgeries, pharmacies and hospital hubs, there will also be a number of larger vaccination sites, including 16 in the Greater Glasgow and Clyde area.

In Northern Ireland, regional centres and GPs' clinics are offering vaccines. Belfast's SSE Arena will also be used as a mass vaccination centre, from April, for those aged 60 and under who have not already been vaccinated.

The UK has ordered more than 400 million doses of seven of the most promising vaccines - three of which have so far been approved for use.

Among them are 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, Health Secretary Matt Hancock has said.

The UK also has significant orders of the Moderna vaccine, which was approved in January.

The UK government has also announced an eighth deal with biopharmaceutical company CureVac to develop vaccines against future variants.

It has placed an initial order for 50 million doses to be delivered later this year - if they are required.

But having vaccines on order is not the same as having them ready to go.

There have been 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.

Design by Lilly Huynh, Irene de la Torre Arenas and Sana Jasemi. Data journalism by Wesley Stephenson, Christine Jeavans and Daniel Dunford.

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