All over-40s in England are now being invited to book their first coronavirus vaccine appointment.
In Northern Ireland, there is also some limited availability for those aged 30 to 34 to book a coronavirus jab.
Appointments are mainly available at the vaccine centre at the SSE Arena in Belfast – with those eligible able to book a slot from 10am today.
The expansion of the vaccine programme comes as figures showed that more than 34 million people across the UK have now received one dose, and more than 14 million people are now fully vaccinated.
Outgoing NHS England chief executive Sir Simon Stevens, who has announced he will step down this summer, 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.
"With second doses also proceeding apace, we’re now ready to invite all those aged 40 and over to join the most successful vaccination drive in health service history."
The target of offering a first dose to the nine most vulnerable groups by 15 April was reached three days early, which allowed the programme to be expanded during the second half of this month.
Despite supply constraints and second jabs for those at highest risk of dying from coronavirus being prioritised, millions more people have still been getting their first inoculations.
Within a week of being invited to book, two-thirds of 45 to 49-year-olds had received their first dose.
Among the 40-somethings to get the jab in recent days is Health Secretary Matt Hancock.
The 42-year-old urged those eligible to join him in getting jabbed after he had his first dose administered by none other than deputy chief medical officer Professor Jonathan Van-Tam.
Mr Hancock said: "The UK’s vaccination programme has been a phenomenal success so far, with more than 47 million doses administered and one of the highest uptake rates in the world.
"Building on this excellent progress we are now opening up vaccinations to 40 and 41 year olds.
"I urge everybody in these age groups to book a jab as soon as possible to protect yourself and your loved ones from this dreadful disease."
Mr Hancock’s appointment was at London’s Science Museum – one of more than 1,700 sites across England helping deliver the vaccine programme.
Others include GP surgeries, pharmacies, places of worship, football grounds and supermarkets.
Meanwhile, in Scotland, preparations continue to extend the vaccine rollout to people in their 40s, while those aged 30 and over are being offered the jab across Wales.
In Northern Ireland, as well as the clinically vulnerable, people aged 35 and over have also been invited to book their vaccine appointment.
Meanwhile, it has been announced that the UK will host a global summit in 2022 alongside a major scientific coalition aimed at supporting plans to accelerate vaccine development in response to future pandemics.
The summit, which will be in partnership with the Coalition for Epidemic Preparedness Innovations (Cepi), aims to raise investment from the international community to support the UK and Cepi’s goal of slashing vaccine development time to 100 days – about a third of the time that it took the world to develop a coronavirus jab.
People now eligible to book a COVID jab in England can do so at a vaccination centre or pharmacy centre through the national online booking service, or by calling 119.
Text invitations appear as an alert from "NHSvaccine" and include a link to the NHS website to book.