Supplies of Pfizer jabs are "tight", the vaccines minister has admitted, while insisting all adults will receive a first dose by the end of July.
On Friday, Scotland’s First Minister Nicola Sturgeon said discussions are taking place between the four nations of the UK about "constrained" supplies of the vaccine.
In many parts of the UK, there have been calls to speed up delivery of the jabs, so the rollout of first jabs can be accelerated among younger groups, while second doses are brought forward for those in middle age.
But supplies of Pfizer/BioNTech are under pressure, following changes made to the rollout last month, when it was decided that those below the age of 40 should be offered an alternative to Astra Zeneca, because of concerns about rare blood clots.
Health officials in England insisted on Friday that there were no shortages of Pfizer, with deliveries arriving as ordered.
The rollout is currently administering first doses to those aged 25 and over, before it moves to the final group of those aged 18 and over, with a target to offer first jabs to all adults by July 31.
Vaccines minister Nadhim Zahawi said the timetable was on track, along with a target to ensure all over 50s have been offered a second jab by June 21.
Delays easing lockdown restrictions could allow more people to have their second dose before they are lifted, with most fortysomethings eligible for second jabs in July.
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Last month the Government shortened the recommended gap between second doses for those aged 50 and over, from 12 weeks to eight.
Ministers are also exploring bringing forward second jabs for those in their 40s.
Most of those in this age group would require Astra Zeneca, but some would need a second dose of Pfizer, where supplies are more constrained, with large numbers of supplies needed for first doses.
Mr Zahawi admitted on Friday that Pfizer vaccine supplies will be "tight" over the next few weeks, but said he is still confident of offering every adult a first dose by the end of July.
Scotland’s Health Secretary had said supplies would be "particularly tight" over the next few weeks, restricting the vaccination programme’s rollout.
Mr Zahawi told LBC he was “absolutely confident” that the UK, including Scotland, would be able to meet its targets for the end of July.
Asked if it would be "tight" in the next few weeks, he said: "It will be, there is no doubt,” insisting that while supply was “finite” it remained stable.
“Pfizer have done a great job in being consistent on their delivery schedule,” he said, saying the company had done "remarkable things to increase their production, not just for us but for the whole world, for Europe and the US as well. But it is tight."
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Pfizer said it had met all its contractual obligations, and was on course to deliver a steady supply of vaccines, in accordance with monthly schedules that had been agreed.
In a statement, the company said: “Decisions on dosing regimens and how best to roll out the vaccination programme to priority groups sit with the health authorities in each country."
Scottish Ms Sturgeon said: "Across all four of the nations, vaccination is going extremely well, but we do know that we have, as we have at points in the past, we have periods coming up where some vaccine supply will be more constrained, and over the next few weeks that looks as if it will be Pfizer.
"Remember Pfizer is now the vaccine we rely on for under-40s, people who had Pfizer earlier on have second doses coming up, so we are simply looking to discuss with all of the four jurisdictions to make sure that we are doing everything we can to maximise the supply we can get from the manufacturers and then use those supplies as effectively and efficiently as possible.”