Mrs Keenan, from Coventry, received the initial injection on December 8, just days before her 91st birthday.
NHS England said at the time that she would have a “booster jab” 21 days later “to ensure she has the best chance of being protected against the virus”.
On Tuesday, she returned to the hospital in her home town where this second dose was administered.
This means she is set to be protected from Covid-19 within days.
90-year-old Margaret Keenan is the person in the UK to get the vaccine
Speaking after receiving her historic first jab, Mrs Keenan – known as Maggie – said: “Hopefully it’ll help other people come along and do what I did, and try and do the best to get rid of this terrible thing.”
The mother-of-two added: “I say go for it, because it’s free and it’s the best thing that’s ever happened.
“If I can do it; well, so can you,” she said.
Professor Andy Hardy, Chief Executive of University Hospitals Coventry and Warwickshire NHS Trust where Ms Keenan received her injection, said: “We were delighted to welcome Margaret Keenan back to Coventry’s University Hospital today to safely receive the second dose of the vaccination, after she became the first person in the world to receive the Pfizer COVID-19 following its clinical approval.
“Our hardworking staff who have been involved in the vaccination programme have remained in contact with Margaret’s family since that day and we are delighted that Margaret has been continuing to recover well at home following her discharge from hospital.
“It’s important that everyone comes forward to get the jab when they are invited to do so and, like other hospitals and GP surgeries across the country, we’ll be following the latest expert advice and evidence to invite people to get vaccinated at the time they need it.”
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The most recent figures published by the Department of Health showed that the number of people to have had their first jab between 8 December and 20 December in the UK was 616,933.
Since the initial jabs were given in hospitals, the rollout has widened to GP-led sites and care homes.
A Welsh Government spokesman said people in Wales would begin receiving their second dose of the vaccine from January 5.
The Pfizer jab, which was developed together with German pharmaceutical company BioNTech, has been authorised for use in more than 45 countries including the UK, the US and across the EU.
The companies submitted data to regulators showing the vaccine, which goes by the brand name COMIRNATY in Europe, is up to 95 per cent effective in preventing infection with Covid-19 when administered in two doses.
The Government has already ordered 40 million doses of the vaccine enough to immunise 20 million people with two shots each.
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However, they have also acknowledged that addtional investigations will need to be carried out to be totally sure.
Meanwhile, a scientist advising the Government has warned that the UK is unlikely to achieve herd immunity through vaccination before the summer.
Calum Semple, professor of outbreak medicine at the University of Liverpool and a member of the Scientific Advisory Group for Emergencies (Sage), described the Oxford/AstraZeneca vaccine as a “game changer” if it is approved by the Medicines and Healthcare products Regulatory Agency (MHRA) in the coming days.
But he told BBC Breakfast on Monday: “To get the wider community herd immunity from vaccination rather than through natural infection will take probably 70 per cent to 80 per cent of the population to be vaccinated, and that, I’m afraid, is going to take us right into the summer I expect.”
The current vaccination target will have to be doubled to two million jabs a week to avoid a third wave of the virus, according to a projection from a London School of Hygiene and Tropical Medicine paper reported by the Telegraph.
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Health Secretary Matt Hancock is due to announce any changes to tier areas in a statement to the Commons on Wednesday.
An expert adviser to the Government said national coronavirus restrictions are needed to prevent a “catastrophe” amid rising infections, and the head of an organisation representing health trusts said “as much of the country as possible” should go into the harshest Tier 4.
The Government said it is “still planning for a staggered opening of schools” at the start of the New Year but is keeping the approach under constant review,
Professor Andrew Hayward, of the Government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) said he thinks schools will have to return “maybe a little bit later” and reopening would mean “we’re going to have to have increased, strict restrictions in other areas of society to pay for that”.
Fellow Nervtag member Professor Neil Ferguson said there had been a “balancing act” since lockdown was initially eased to try to keep control of the virus while maintaining “some semblance of normal society” but that the new variant had made it more difficult.
He told BBC Radio 4’s World At One programme on Tuesday: “Clearly nobody wants to keep schools shut. But if that’s the only alternative to having exponentially growing numbers of hospitalisations, that may be required at least for a period.”