EU regulators also acknowledged a link between unusual clotting and the coronavirus shot in "very rare" cases, but upheld their endorsement of the drug's use in all adults.
The British Medicines and Healthcare products Regulatory Agency (MHRA) and Joint Committee on Vaccination and Immunisation (JCVI) said people aged 18 to 29 who do not have an underlying health condition will now be recommended other vaccines instead.
Those vaccines include the Pfizer/BioNTech or Moderna jabs, which have both been approved for use alongside the AstraZeneca/Oxford vaccine and have begun being rolled out in Britain.
Links with brain thrombosis cases
In contrast, the European Medicines Agency confirmed its recommendation to use the vaccine in all adults despite also finding links of very rare cases of brain thrombosis linked to the coronavirus jabs.
Though unusual blood clots should be listed as a possible side effect, the benefits of the jab still outweigh the risks, the EMA explained following a review of cases.
"Covid-19 is a very serious disease with a high hospitalization and death rate, and everyday Covid is still causing thousands of deaths across the EU," EMA director Emer Cooke said at a press conference.
The announcement confirms a previous assessment by the agency, which had deemed the vaccine safe and effective.
The British bodies added there was not enough evidence to show that the vaccine causes clotting and there were still huge benefits of the vaccine in preventing Covid-19, stating that people should continue to get their jabs.
Those who have had their first dose of the AstraZeneca/Oxford vaccine but have not yet had their second dose are being told to get their second dose as normal.
Anyone with blood disorders that leave them at risk of clotting should discuss the benefits and risks of vaccination with their doctor before going for a jab, they added.
79 reports of blood clotting cases
The MHRA and the JCVI announced the move in a joint press conference after the EMA's press conference on Wednesday.
It came after the MHRA received 79 reports of blood clotting cases alongside platelet low levels following the use of the AstraZeneca/Oxford vaccine in Britain.
Fifty-one women and 28 men, aged from 18 to 79 years, reported the clotting.
Out of the 79 cases, 19 people died in total - 13 women and six men.
Each person who died had either cerebral venous sinus thrombosis with thrombocytopenia - which stops blood draining from the brain properly - or thrombosis with thrombocytopenia, which occurs in major veins.
More than 18 million people have received a dose of the AstraZeneca/Oxford vaccine in Britain so far.
Head of the MHRA, Dr June Raine, said the risk of blood clotting in people who receive the AstraZeneca/Oxford vaccine is about four people in a million and the benefits outweigh the risks for the vast majority.
Professor Wei Shen Lim, chairman of the JCVI, added: “We are not advising a stop to any vaccination for any individual in any age group.
“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.”
Cases reviewed by EMA experts
EMA experts reviewed 62 cases of blood clots in veins in the brain, and 24 in veins in the abdomen, with 18 of these being fatal. Around 25 million people had received the vaccine in European Economic Area and Britain, EMA noted.
“When millions of people receive these vaccines very rare events can occur that were not identified during the clinical trials,” Cooke told reporters at a live streamed press conference.
Based on the evidence so far, it is not possible to confirm whether age, gender, or a previous history of clotting disorders are risk factors, she said.
EMA’s Pharmacovigilance Risk Assessment Committee “can therefore not recommend any specific measures to reduce the risk,” committee chair Sabine Straus said.
AstraZeneca will have to look into the reactions itself, and EMA has commissioned its own research, Straus added.