People under the age of 30 in the UK will be offered an alternative to the AstraZeneca vaccine amid concerns the jab causes blood clots.
They will instead be offered the Pfizer or the new Moderna vaccine.
England's deputy chief medical officer Professor Jonathan Van-Tam hosted a briefing with the medicines regulator following an investigation into the Oxford/AstraZeneca vaccine and potential links with blood clots.
Prof Van-Tam was joined by Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), Professor Sir Munir Pirmohamed, chairman of the Commission on Human Medicines, and Professor Wei Shen, chairman of the Joint Committee on Vaccination and Immunisation (JCVI).
Dr Raine warned that "no effective medicine or vaccine is without risk", and said that "in an extremely small number of people", there was evidence of blood clots as a potential side effect.
Up to Mar 31, there were 79 cases of people developing blood clots having received an AstraZeneca vaccine.
Of those, 19 died, and three of those who passed away were under the age of 30.
But Dr Raine stressed that after 20million doses of the vaccine have been administered, the chances of someone developing a blood clot having had the jab remains very small - four in one million.
She also added that while it was "a strong possibility" the clots were caused by the vaccine, "more work is needed" to make absolutely sure this was the case.
Prof Wei Shen of the JCVI outlined three changes to Government advice regarding the vaccine rollout.
Information to individuals being offered vaccination and to health professionals should be appropriately updated to reflect the latest considerations from the MHRA and JCVI
Those who have received the first dose of the AstraZeneca vaccine should continue to be offered the second dose according to schedule
Adults aged 18-29 years (who do not have any underlying health condition that put them at higher risk of serious Covid-19 disease) should be offered an alternative Covid-19 vaccine in preference to the AstraZeneca vaccine, when an alternative is available
A televised media briefing on Wednesday came at the same time the European Medicines Agency (EMA) offered its own update on whether it thinks there is a causal link between the jab and rare clots.
The European regulator concluded that "unusual blood clots with low blood platelets should be listed as very rare side effects" of the Oxford/AstraZeneca vaccine.
It noted, however, that the benefits of being vaccinated outweighed the risk of any side effects.
The EMA and the MHRA have both carried out reviews into reports of rare brain clots in people who have had the AstraZeneca vaccine.
What have the MHRA said?
The MHRA today said that by March 31, there had been 79 reports of clots linked to the AstraZeneca jab in Britain, in more than 20 million doses and 19 deaths.
Dr June Raine, MHRA Chief Executive, said the benefit continued to outweigh the risk: "Vaccines are the best way to protect people from Covid-19.
"But no effective medicine or vaccines is without risk, and with vaccines more complex than usual because the benefits can be for other than those taking the vaccine,” she told a televised press briefing on Wednesday afternoon.
"Very rare effects are only detected when a vaccine is used at scale, which is why the UK has careful monitoring systems. These monitoring systems are not detecting a potential side effect in an extremely small number of people.
"While it’s a strong possibility more work is needed to establish beyond all doubt that the vaccine has caused these side effects. The public’s safety is at the forefront of our minds."
What has the CHM said?
Talking on behalf of the Commission of Human Medicines, Professor Sir Munir Pirmohamed outlined three groups who should seek advice before having the AstraZeneca vaccine.
Pregnant woman should continue to discuss with their healthcare professional to discuss whether the benefits of having the vaccine outweigh the risks for them
People with a history of blood disorders that increase the risk of clotting should only have the Covid-19 vaccine AstraZeneca when the benefits outweigh any potential risks
Anyone who experienced cerebral, or other major blood clots occurring together with low levels of platelets
Sir Munir added: "Just to put into context, these events are extremely rare, and I want to put it into context, in relation to Covid-19 itself.
"It is important to remember the Covid-19 itself causes clotting and it causes lowered platelets.
He said the vaccine "remains favourable for the vast majority of people" but "is more finely balanced for the younger people".
Closing his statement, Sir Munir said: "And we are advising that this evolving evidence should be taken into account when considering the use of the vaccine."
What has the JCVI said?
Professor Wei Shen Lim, chairman of the Joint Committee on Vaccination and Immunisation, set out the recommendation to offer an alternative jab in preference to AstraZeneca for the under-30s.
"We are not advising a stop to any vaccination for any individual in any age group," he said.
"We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution rather than because we have any serious safety concerns."
He acknowledged the shift would have "some implications" in terms of the deployment of vaccines.
Professor Wei Shen recommended those who are just over the age in which the Oxford/AstraZeneca jab will no longer be offered should make their own decision but insisted taking it is the safer option.
He told the briefing: "For somebody who is 31 or 32 I think they have to make their own decision as to what they want to do about vaccination.
"We would still say that the balance is in favour of being vaccinated because of the risk from Covid-19 and the protection the vaccine offers."
What is the context?
Jonathan Van Tam, the deputy chief medical officers, said that when the virus was low in the community the risks for under 30s was higher from the vaccine than from coronavirus. But he warned that if the prevalence rose, then the benefit would also rise.
“We’re talking in the context of extremely small numbers,” he said.
“The UK vaccine programme has been the most enormous success. We must keep this in context of the enormous success we’ve achieved so far.
“This is a course correction there is no question about but it’s quite normal for physicians to alter their preferences over time.
“This is a massive beast that we are driving along at enormous pace and enormous success. And if you sail a massive liner across the Atlantic then it’s not really reasonable that you aren’t going to make at least one course correction on that voyage.”
Why is there concern over the AstraZeneca vaccine?
All the major European countries suspended the use of the AstraZeneca jab after a flurry of nations announced temporary halts to their programmes last week.
Norway and Austria were the first to sound the alarm - Austria reported that a person was diagnosed with blood clots and died 10 days after they had received the vaccination. Another was hospitalised with pulmonary embolism (blockage in arteries in the lungs) after being vaccinated.
Then Denmark announced that someone had died after receiving the jab and became the first country to suspend its AstraZeneca programme.
This prompted other countries, including Norway, the Netherlands and Ireland to follow suit. Thailand also suspended its programme but has now restarted it.
Mr Macron prompted an outcry earlier this year when he claimed the jab was "quasi-ineffective" for pensioners.
In February, Mrs Merkel, 66, faced heavy criticism for refusing the AstraZeneca jab, insisting it was not recommended for people over 65.
In her first major interview, Kate Bingham, the former head of the Government's vaccine taskforce, accused the French president and German chancellor of encouraging anti-vaxxers after both cast doubt on the efficacy of the vaccine, developed by British scientists.
Bingham's criticism comes as France and Germany currently face a third wave of coronavirus cases.
What does AstraZeneca say?
Previously, the company has reviewed its safety data and carried out further testing but has found no links to clotting.
The company’s chief medical officer Ann Taylor has said the number of cases of blood clots reported in the 17 million people across Europe who have received the vaccine is actually lower than would be expected in the general population.
She added: “The nature of the pandemic has led to increased attention in individual cases and we are going beyond the standard practices for safety monitoring of licensed medicines in reporting vaccine events, to ensure public safety.”