People aged 40 and over in England are now able to book their Covid jabs, NHS officials say.
Text messages will be sent to 40 and 41-year-olds, directing them to the national booking service.
Meanwhile, about 22 million people in the UK are living in areas that have not reported any Covid deaths that happened in April, BBC analysis shows.
Since the vaccine rollout began in December, about 34 million people have had at least one dose in the UK.
This equates to nearly two-thirds of the adult population. More than 14 million people have now had two Covid jabs.
The extension of the rollout comes just days after vaccinations were opened to 42-year-olds in England.
NHS England chief executive Sir Simon Stevens said: "With nine-tenths of people aged 45 and over having been jabbed, nearly three-quarters of a million new appointments were made in just two days as our booking service opened to people aged 42 to 44."
It comes as fewer than 600 deaths within 28 days of a positive test have been reported so far this month, compared with more than 30,000 in the first four weeks of January.
The latest lockdown, together with the vaccination programme, have successfully reduced cases, and as a result deaths.
Dr Mike Tildesley, an expert in the spread on infectious diseases, said it was "really good news".
"It's a really good sign and we've seen several parts of the country where prevalence is really, really low. So, I think it gives us confidence."
But he echoed another expert who urged caution.
"[We] have seen before, when things go wrong they can go wrong quickly," said Prof Graham Medley, of the London School of Hygiene and Tropical Medicine.
Lifting restrictions slowly means this is less likely to happen, he added.
People who cannot access the booking service online can call the service on 119 to book a vaccination appointment.
Northern Ireland has opened its vaccination programme to 35 to 39-year-olds.
Wales, meanwhile, has been offering jabs to the over-40s, as well as younger people in some areas.
And Scotland has been inviting 45 to 49-year-olds.
The government plans to offer a first vaccine dose to the rest of the adult population by the end of July.
The NHS currently uses three vaccines against coronavirus:
And the government has ordered an extra 60 million doses of the Pfizer-BioNTech vaccine as part of its plans for a vaccination booster programme in the autumn.
It comes as Liverpool prepares to host a government-organised live music event as part of a Covid safety pilot to check what impact easing the rules will have ahead of a full lifting of restrictions later in the year.
An indoor club night for around 3,000 people will take place at Circus Nightclub with no social distancing and no face coverings under the government's Events Research Programme (ERP), which will explore the risk of transmission and the effectiveness of measures such as ventilation and testing.
Meanwhile, shops, gyms, pubs, restaurants and cafes in Northern Ireland will reopen on Friday after a four-month winter lockdown.
Hospitality businesses must only operate outdoors, with table service and limited numbers per table. Caravans and self-contained tourist accommodation can also now reopen.
Restrictions on meeting up outside have also been slightly further eased, with 15 people from three households allowed to meet in a private garden.
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