Britons aged 18-29 will be offered an alternative to the Oxford-AstraZeneca vaccine after 79 people developed blood clots within days of getting the jab.
There is a possible link between the Oxford vaccine and "extremely rare and unlikely to occur" blood clots with lowered platelets, the Medicines and Healthcare Products Regulatory Agency (MHRA) has concluded.
Younger people are much less likely to die from COVID-19, so the Joint Committee on Vaccination and Immunisation (JCVI) has decided it is safer to advise that age group are offered a different jab, where possible.
England's deputy chief medical officer Professor Jonathan Van-Tam said the new advice is a "course correction" for the UK's "very successful" vaccine rollout - and said for most age groups the "benefits outweigh the risks".
He said the new advice will have a negligible impact on the UK's rollout, which is continuing "full speed ahead".
Oxford Vaccine Group chief Andrew Pollard said the identification of the blood clot cases "shows the safety system works".
The advice is being given after a total of 79 people in the UK have developed blood clots following their first Oxford-AstraZeneca jab up to the 31 March, Dr June Raine, chief executive of the MHRA, said.
More than 20 million people have been given the Oxford vaccine.
Of those who have developed blood clots, 19 have died - three under the age of 30.
A total of 51 women and 28 men aged 18 to 79 were affected, but Professor Sir Munir Pirmohamed, chair of the commission on human medicines, said there is no evidence women have a predilection to develop blood clots after having the Oxford jab.
"The risk is four people in a million," Dr Raine added.
Both UK and EU regulators have requested Astrazeneca lists the "extremely rare potential side effect" on the vaccine's labels, the pharmaceutical giant said. It added that it has been "actively collaborating" with the regulators.
Dr Raine said anybody suffering the following side effects four days after getting a jab should seek medical attention:
Shortness of breath
Bruising or pinpoint spots beyond the vaccination site
The JCVI has said people of any age who have received the first dose of the Oxford vaccine should continue to be offered the second dose according to schedule.
JCVI chairman Professor Wei Shen Lim said: "We are advising a preference of one vaccine over another vaccine for a particular age group out of utmost caution rather than any serious safety concerns."
He added that people who are just over 29-years-old should make their decision, but getting the vaccine is much safer than not getting it.
Prof Van Tam said it is quite usual for physicians to alter their preference on medicines and vaccines, and said the NHS will get the "right vaccine to you at the right time" but said some people may have to travel further for their vaccines.
He said it remains "vitally important" that people get their vaccine when they are invited.
Currently, the UK is also rolling out the Pfizer jab, and the first doses of the Moderna vaccine were administered today in Wales. Mr Van Tam said Moderna jabs are expected in England in mid-April.
Following the announcement, Boris Johnson reiterated that "this vaccine is safe, effective and has already saved thousands of lives - and the vast majority of people should continue to take it when offered".
The development comes as the EU's medicines regulator announced the conclusions of its own review, saying that "unusual blood clots" should be listed as a "very rare" side-effect.