People aged 56 to 59 in England are being invited to book in for a COVID jab, as the NHS vaccination programme continues to gather pace.
Letters sent to 850,000 people in the age bracket began arriving on Saturday, with another 850,000 expected to arrive on Monday.
It comes after more than eight in 10 people aged between 65 and 69 took up the offer of a jab.
More than 18 million people across England – more than one third of the adult population – have already received a vaccine.
Across the whole of the UK, 22,887,118 COVID-19 jabs have been given so far, according to government data up to 5 March.
Of these 21,796,278 were first doses and some 1,090,840 were second doses.
On Friday, Health Secretary Matt Hancock said more than two-fifths, or 40%, of the UK adult population had been offered a vaccine dose.
Ministers say the UK is on track to vaccinate every adult at least once by the end of July.
Before then, the government is aiming to vaccinate everyone in the first nine priority groups on the Joint Committee on Vaccination and Immunisation (JCVI) schedule – equivalent to 32 million people – by 15 April.
This includes the clinically vulnerable and all over-50s.
Vaccinations are being administered at more than 1,600 sites across the country, including mosques, museums and rugby grounds.
Mr Hancock said: “The NHS vaccination programme is in full swing and more than 21 million of the most at-risk people have already received their first dose.
“This includes more than 90% of those aged 65 and over and almost nine in 10 people who are clinically extremely vulnerable.
“It’s an extraordinary feat and we remain on track to offer a first vaccine to all adults by 31 July.
“We are now inviting people aged 56 to 59 to get their jabs and I urge everyone eligible to come forward as soon as possible.”
Dr Nikki Kanani, NHS national medical director for primary care and a GP, said: “The NHS vaccine programme is committed to protecting the country against coronavirus and it is testament to our incredible staff that we can now move on to the next age group.”
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People receiving the letters are invited to go book a vaccination slot via the national booking service If they are not able to go online, they can call 119 free of charge.
The NHS made history when Maggie Keenan became the first person in the world to be protected against coronavirus outside of a clinical trial when she received the Pfizer vaccine at Coventry Hospital on 8 December.
The NHS was also the first to deliver the new Oxford AstraZeneca coronavirus vaccine when Brian Pinker, 82, received his on 4 January.