People aged between 56 and 59 are now being invited to book coronavirus vaccinations, NHS England said.
From Saturday, hundreds of thousands of letters are beginning to land on the doorsteps of the age group.
The latest round of invites comes as NHS England says eight in 10 people aged 65-69 took up the offer of a jab.
So far, more than 18 million people across the country have already received a jab - more than one third of the adult population.
And across the whole of the UK, more than a million people have received both doses of a Covid-19 vaccine, while almost 21.4 million people have had one dose.
Dr Nikki Kanani, NHS England national medical director for primary care, 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.
“We must not forget that our success in vaccinating more than 18 million people is down to careful planning and the hard work of those staff.
“The vaccines are both safe and effective, so if anybody who is eligible but hasn’t been vaccinated yet, I’d urge them to go online or call 119 and get themselves booked in.”
Health Secretary Matt Hancock said more than nine in 10 of those aged 65 and over and nearly nine in 10 people who are clinically extremely vulnerable have received a first dose.
He said: “It’s an extraordinary feat and we remain on track to offer a first vaccine to all adults by July 31.
“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.”
NHS England added: “Vaccinations are now being administered at more than 1,600 sites across the country, including mosques, museums and rugby grounds, with the distribution of centres meaning 98% of the country lives within 10 miles of at least one vaccination service.
“People can use the national booking service to reserve a slot at a NHS vaccination centre or one of the pharmacy-led services across the country.”
People in England can book a coronavirus vaccine on the NHS website here.
