Only three Britons have contracted the Indian variant of Covid-19 after having both jabs, Health Secretary Matt Hancock said.
It comes as all over-25s in England will be able to get a Covid vaccine from today.
Mr Hancock announced the change in Parliament as he said Britain’s rollout is moving at an ‘extraordinary pace’.
Until now only over-30s had been able to book appointments, save for surge clinics in some areas where they had been temporarily opened to younger groups.
People in their 20s are the adults at lowest risk from Covid but the NHS has now given a first dose to 77 per cent of people already so only around one in four are left.
The chief of the NHS, Sir Simon Stevens, said England is now entering the ‘home straight’ of the vaccine programme and added ‘getting the jab is the most important thing you can do’.
Britain is now in a race against the Indian variant as medics try to vaccinate as many people as possible to protect them from the strain which Mr Hancock said is likely 40 per cent more transmissible than the Kent variant was.
In a boost for vaccine effectiveness, just three people who have had both vaccine doses were admitted to hospital with the Delta variant.
‘The jabs are working, we have to keep coming forward to get them and that includes, vitally, that second jab, which we know gives better protection against the Delta variant,’ Mr Hancock said.
Public Health England warned last week a single dose of vaccine does not appear to work as well against the now-dominant ‘Delta’ variant as it did against the Kent strain, meaning getting the booster jab is crucial.
Scientists have said as many people as possible – including teenagers – need to get vaccinated to stop the new variant from spreading.
Meanwhile, Wales’s First Minister Mark Drakeford said the NHS there will have offered vaccines to all adults over 18 by Monday, putting it six weeks ahead of the end-of-July schedule.
The Health Secretary announced the change in Parliament yesterday as scientists warned jabs must be given to as many people as possible to stop the new variant
Mr Hancock said in the House of Commons: ‘From this week we will start offering vaccinations to people under 30, bringing us ever closer to the goal of offering a vaccine to all adults in the UK by the end of next month.
‘From tomorrow morning we will open up vaccination to people aged 25 to 29.
‘Over the remainder of this week the NHS will send texts to people in these age groups and of course GPs will be inviting people on their lists to come forward…
‘The latest estimates indicate the vaccination programme has averted over 39,000 hospitalisations and over 13,000 deaths.
‘So the vaccination brings us hope and I’m sure the whole house will join me in thanking people for their perseverance and patience as they’ve waited for their turn.’
Sir Simon added: ‘Tomorrow is a watershed moment as the world-beating NHS vaccination programme enters the home straight of our race to offer everyone their first dose.
‘The NHS vaccination programme is a real team effort and it is a testament to NHS teams across the country, that we are able to open up to people in their twenties just six months on from delivering that world first jab to Maggie Keenan.
‘The tireless efforts of NHS staff to protect the people they care for and their communities has quite simply saved lives, and when you get the text, you’re next.’
The vaccine rollout has given 40.3million people across the UK their first dose and 27.9million both jabs, meaning half of all adults are fully vaccinated.
Mr Hancock said the vaccines have saved thousands of lives already and kept even more people out of hospital.
It is rare for people who have had both doses to end up in hospital or die, even after catching the Indian variant, he said.
Scientists’ and ministers’ biggest fear is that a new variant that the vaccines don’t work against will come along and cause a gigantic third wave – but the current Delta strain is still susceptible to jabs.
The Health Secretary said the vaccine is ‘breaking the link between infections, hospitalisations and deaths, a link that was rock solid back in the autumn’.
Cases in Bolton, the first hotspot for the strain, have started to fall, he said as he attributed some of this success to surge vaccinations, offering proof that the variant could be controlled without lockdowns.
He added: ‘Despite the rise in cases, hospitalisations have been broadly flat.
‘The majority of people in hospital with Covid appear to be those who haven’t had the vaccine at all.
‘I want to update the House on some new information that we have on this. As of the third of June our data show that of the 12,383 cases of the Delta variant, 464 went on to present at emergency care and 126 people were admitted to hospital.
‘Of these 126 people, 83 were unvaccinated, 28 had received one dose and just three had received both doses of the vaccine.’
He said: ‘The jabs are working, we have to keep coming forward to get them and that includes vitally that second jab which we know gives better protection against the Delta variant.’
Wales today committed to offering jabs to all over-18s by Monday.
First Minister Mark Drakeford said the milestone will be hit six weeks ahead of the UK target for giving vaccines to everyone over the age of 18 by the end of July.
Wales has given a first jab to 2.18million people, or 86 per cent of the adult population, which is currently the best record of any country in the world.
However, the country is lagging behind England and Scotland when it comes to the proportion of the population who have received both doses.
Mr Drakeford told a press conference in Cardiff: ‘We will make the offer of vaccination to all eligible adults six weeks ahead of schedule and we expect to reach 75 per cent take-up across all priority groups and age groups a month ahead of target.
‘This is a remarkable achievement and a tribute to the hard work of all those involved in the programme – to all those doing the complex work of planning behind the scenes and to the thousands of people vaccinating and helping to run the clinics across the country.’
He said the government would now ‘switch our efforts to accelerate second doses’, with a target of covering the whole population by September.
‘Subject to supply, we’re confident that we can deliver second doses as fast and as successfully as we have first doses,’ he said.
The updated strategy expects to deliver around 28,000 second doses every day over the summer months.