More than 31 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 easing a third national lockdown on vaccinating as many adults as possible by summer.

But vaccine supply issues and have continued to make the rollout bumpy.

The UK government aims to offer a first vaccine dose to about 32 million people in nine priority groups by 15 April.

The programme in England is now inviting those aged 50 and above to book appointments after the first four groups - those aged 70 and over, care home residents, healthcare workers and people required to shield - were offered a jab by mid-February.

These groups account for 88% of deaths so far.

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The over 55s in Scotland, the over 50s in Northern Ireland and the over 50s in Wales have been asked to book appointments.

The government then plans to begin vaccinating the rest of the adult population in age order, another 21 million people.

People in their 40s will be next, once the current phase is completed.

More than 31.5 million people have had a first vaccine dose and more than 5.4 million have had a second.

The number of first doses administered each day is now averaging just over 250,000 - a drop from an average of about 500,000 in mid-March as the schedule of second doses starts to kick in.

The government has often described vaccine supplies as "lumpy" and the UK is expected to see a reduction in doses available during April.

Health Secretary Matt Hancock told MPs the drop had been caused by the need to re-test the stability of 1.7 million doses as well as a delay in the scheduled arrival of jabs from the Serum Institute of India.

Despite the expected dip in supplies in April, the government says the country is still on track to offer a first dose to everyone aged 50 and over by the end of April, and to all adults by the end of July.

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

There continues to be some regional variation in the vaccine programme.

England, on average, has vaccinated 93% of those aged 50 and over, with the South West reaching 96% of people in that age group and London 85%.

In Scotland, 90% of over 50s have had at least one dose of the vaccine, while Wales has reached 82% and Northern Ireland 59%.

When looking at all those aged 18 and above, England, Scotland, Wales and Northern Ireland have all vaccinated more than half of adults with the first dose.

Second doses are also being rolled out. Wales has administered the second dose to 18% of adults, driven by the prioritisation of vaccines to those working in the health and care sectors.

There have also been 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 four of the older age groups.

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, 79% of those aged between 60-64 in the poorest areas had been given a vaccine by 25 March compared with 91% 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.

Further doses are expected to come from the Serum Institute of India and the Halix plant in the Dutch city of Leiden.

Supplies of a third vaccine to be approved, made by US company Moderna, will come from Europe.

Scotland's chief medical officer, Dr Gregor Smith, told the Scottish government's daily Covid briefing on Thursday that Moderna supplies could arrive next week. He said the jab was similar to the Pfizer vaccine, which means it needs ultra-cold storage and will only be available at the very large vaccination centres in Edinburgh, Glasgow and Aberdeen.

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.

Some vaccine centres will close temporarily in April as supply shortages kick in, while others will stop taking bookings for first dose appointments.

The UK has ordered more than 400 million doses of seven of the most promising vaccines.

Three have so far been approved for use: Oxford-AstraZeneca; Pfizer-BioNTech; and Moderna.

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 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, checking those supplies are up to scratch and transporting vaccines according to their requirements have all thrown up difficulties.

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