The EU's drug regulator said yesterday that blood clots should be listed as a "very rare" side effect of the AstraZeneca coronavirus vaccine but that the jab's benefits continue to outweigh the risks.

No specific risk factors, including age, have been identified for thrombosis with the AstraZeneca shot, which could stem from an immune response, the European Medicines Agency (EMA) said.

The watchdog's findings come after several countries halted the use of the vaccine following dozens of cases of people with clots in blood vessels draining from the brain after receiving jabs, some of them fatal.

"EMA's safety committee has concluded today that unusual blood clots with low blood platelets should be listed as very rare side effects" of the AstraZeneca jab, the Amsterdam-based watchdog said in a statement.

But it stressed that it believed people should continue to take the vaccine as part of the battle against the disease.

"The safety committee has confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risk of side effects," EMA chief Emer Cooke told a news conference.

"It is saving lives."

Despite the fact that many of the cases have been reported in women under 55, prompting a number of countries to restrict the vaccine's use to older people, the regulator said it had not been able to pinpoint those at risk.

"Specific risk factors such as age, gender or medical history have not been able to be confirmed, as the rare events are seen in all ages," Cooke said.

"A plausible explanation for these rare side effects is an immune response to the vaccine."

Meanwhile, a British government committee advising on coronavirus vaccinations said yesterday most people under 30 should be offered an alternative to the AstraZeneca jab if possible, due to concerns over blood clots.

"Adults who are aged 18 to 29 years old who do not have an underlying health condition... should be offered an alternative Covid-19 vaccine in preference to the AstraZeneca vaccine, where such an alternative vaccine is available," Wei Shen Lim of the Joint Committee on Vaccination and Immunisation, said at a press conference.

Canada, France, Germany and the Netherlands are among several countries that are not recommending the shot for younger people, although the World Health Organization insists the benefits of the jab largely outweigh the risks.

The University of Oxford said on Tuesday it had paused a small UK trial testing the Covid-19 vaccine it developed with AstraZeneca Plc in children and teenagers, as it waits for more data on rare blood clotting issues in adults who received the shot.

There were no safety concerns in the pediatric trial, Oxford University said, adding that it would await guidance from the UK drugs watchdog before giving any further vaccinations.

The controversy surrounding the jab has marred a global vaccine rollout that governments hope will help countries emerge from a pandemic that has ravaged the global economy and subjected much of humanity to some form of confinement.