People will be able to get a COVID-19 booster on Christmas Day as part of NHS England's "jingle jab" campaign.
The rollout will also continue on Christmas Eve and Boxing Day, with 200,000 appointments still available across the three-day period.
NHS staff and volunteers are set to administer the doses at local vaccination sites including town halls and local pharmacies across England.
It comes as Boris Johnson urged people to get the "wonderful" gift of a "first, second or booster" jab as part of the "neighbourly" spirit of the season in his Christmas message.
Those who have not booked in advance will be able to drop into sites across the nation.
Dr Emily Lawson, head of the NHS COVID vaccination programme, said: "This Christmas, before sitting down to your dinner with your family, I would encourage anyone not already boosted to come forward, book an appointment and get the gift of a jab."
But concerns have been raised about the effectiveness of the booster campaign after it emerged that the UK, EU and US have received more doses of COVID-19 jabs in the last six weeks than African countries have in all of 2021.
Only 8.6% of people in Africa have been fully vaccinated so far and if the current rate of delivery by vaccine manufacturers continues, it will not be until April 2023 that everyone will receive their first dose, according to the People's Vaccine Alliance.
Vaccine equity campaigners say that governments like the UK risk "trapping the world in an endless cycle of variants, boosters, restrictions and even lockdowns" if more people are not vaccinated in the global south.
From 11 November to 21 December, the EU, UK and US received 513 million doses of vaccines while countries in Africa received 500 million throughout the year.
G7 countries are on course to have 1.4 billon extra doses by March 2022, even after giving all adults a booster - and yet they are failing to deliver on donation pledges.
The UK has delivered only 15% of what it promised, while the figures for the US and Germany are about 25% and 14% respectively.
The People's Vaccine Alliance is also calling on Pfizer and Moderna to share their vaccine recipes with poorer countries so they can manufacture their own vaccines.
Anna Marriott, health policy manager at Oxfam and the People's Vaccine Alliance, said that preventing vaccine access in poorer countries is "prolonging the pandemic and all its suffering for every one of us".
She added: "Rich countries are banking on boosters to keep them safe from Omicron and future variants of COVID-19.
"But boosters can never be more than a temporary and inadequate firewall.
"Extinguishing the threat of variants and ending this pandemic requires vaccinating the world."
Latest analysis by the UK Health Security Agency (UKHSA) has suggested that Omicron is milder than past COVID variants such as Delta, with between 50% and 70% of people less likely to be admitted to hospital.
But Health Secretary Sajid Javid has warned that Omicron still has the potential to overwhelm the NHS despite the "promising" data because it is more infectious than past variants.
The UK experienced yet another record-breaking number of daily reported COVID cases, with 119,789 reported as of 9am on Thursday.
This was the second day in the whole of the pandemic that daily lab-confirmed case rates were above 100,000, after Wednesday.