People under 40 will be will be offered an alternative to the Oxford/AstraZeneca jab where possible, it has been announced this morning.
It follows "extremely rare" instances of blood clots after receiving the jab - but officials maintained that the public should have "full confidence" in the vaccine programme.
Up to this point, the advice was only for people under 30 to be offered an alternative vaccine.
Professor Jonathan Van-Tam, England's deputy chief medical officer, today said the move would not affect the speed of the rollout - saying he was confident all adults would still be offered a first jab by the end of July.
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA) said the public can still have "full confidence" in the vaccine programme.
She said that rates of reported blood clots were 10.5 per million after a first dose of the AstraZeneca jab, and one per million after the second.
As of last night, 34,934,171 people in the UK had received a first dose of one of the approved vaccines - 66.3 per cent of the adult population.
Of these 16,291,719 had been given both doses, meaning 30.9 per cent of adults in the UK have been fully vaccinated.
In a televised briefing today, Prof Van-Tam said it is crucial that the pace of the vaccination programme continues.
He said: "The point has been emphasised several times already this morning about maintaining the pace and the volume of the vaccine rollout in order to control the disease in our country, long term, and to minimise the impact of any upsurge in cases potentially associated with a third wave.
"I can't overemphasise that enough. I can say to you on top that on current plans, our vaccine supply schedule will support the change offered by the JCVI (Joint Committee on Vaccination and Immunisation) without limiting the speed and scale of the vaccine rollout.
"I do expect that we are still on target to offer a first dose to all adults by the end of July."
He said "some very fine minds" had made the decision "in exactly the right way".
Professor Wei Shen Lim, Covid-19 chair for the Joint Committee on Vaccination and Immunisation (JCVI), said: "As Covid-19 rates continue to come under control, we are advising that adults aged 18-39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine.
"The advice is specific to circumstances in the UK at this time and maximises use of the wide portfolio of vaccines available."
The (JCVI) said there are no new safety concerns after previous data linked it to very rare blood clots.
The Medicines and Healthcare products Regulatory Agency (MHRA) says the balance of risk for the AstraZeneca vaccine against Covid is very favourable for older people but "more finely balanced" for younger groups - who do not tend to suffer serious coronavirus illness.
Up to April 28, the MHRA had received 242 reports of blood clots accompanied by low blood platelet count in the UK, all in people who had AstraZeneca, out of around 28.5 million doses given.
These clots occurred in 141 women and 100 men aged from 18 to 93, and the overall case death rate was 20 per cent, with 49 deaths.
Six cases have been reported after a second dose of the vaccine.
Professor Lim said: "Safety remains our number one priority.
"We have continued to assess the benefit/risk balance of Covid-19 vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.
"As Covid-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine.
"The advice is specific to circumstances in the UK at this time and maximises use of the wide portfolio of vaccines available.
"The Covid-19 vaccines have already saved thousands of lives and the benefit for the majority of the population is clear - if you are offered the vaccine, you should take it."
A particular type of brain blood clot - cerebral venous sinus thrombosis (CVST) - was reported in 93 cases (with an average age of 47), and 149 had other major thromboembolic events (average age 55) accompanied by low blood platelet count.
The MHRA and JCVI have both said that the benefits of the AstraZeneca vaccine continue to "outweigh the risks for the vast majority of adults".
