Under-30s should be offered alternative Covid-19 vaccines rather than the Oxford/AstraZeneca jab where possible, government advisers have recommended.

The Joint Committee on Vaccination and Immunisation (JCVI) announced the decision on Wednesday, amid reports of very rare incidences of blood clotting that may be linked to the jab.

JCVI chair Professor Wei Shen said the recommendation to prefer other vaccines to AstraZeneca for the under-30s was “out of the utmost caution” rather than because of “any serious safety concerns”.

And chief executive of the MHRA Dr June Raine told a press conference: “While [the link is] a strong possibility, more work is needed to establish beyond all doubt that the vaccine has caused these side effects.”

The body has said it identified 79 cases of rare blood clot events after the first dose out of 18.1m doses of the jab administered up to and including March 31.

Of the 51 women and 28 men affected, 19 died. The risk is equivalent to four people in one million. 

The JCVI has said those aged between 18 and 29 should be offered an alternative vaccine – either the Pfizer or the Moderna jabs.

It comes after a review by the European Medicines Agency’s (EMA) safety committee concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the vaccine – but that the benefits of the vaccine still significantly outweighed the risks.

The EMA’s executive director Emer Cooke said: “The EMA’s expert committee on the safety of medicines (PRAC) has confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risks of side effects.”

She added: “A plausible explanation for these rare side events is an immune response to the vaccine.”

The EMA said most of the cases of blood clots reported have occurred in women under 60 within two weeks of vaccination with the AstraZeneca jab, but that no specific risk factors had been identified based on current evidence.

The news means the rollout of the vaccination programme could be slowed significantly as more than a fifth of the UK’s vaccine supply is tied up in the Oxford/AstraZeneca jab.

The government has secured a total of 457m doses, of which 100m are from AstraZeneca. 

Despite this, the government has recommitted to its pledge to offer all adults a Covid-19 vaccination by summer, with the first doses of the Moderna vaccine rolled out on Wednesday. Prime minister Boris Johnson said he did not believe the new advice would force him to alter the schedule for easing the lockdown. 

During a visit to Cornwall, he said: “I don’t think anything that I have seen leads me to suppose that we will have to change the road map or deviate from the road map in any way.”

Johnson sought to reassure the public the Oxford/AstraZeneca vaccine is safe.

He told reporters: “These vaccines are safe, they’ve saved many thousands of lives and people should come forward to get their jabs and we’ll make sure that they get the right jabs.”

Health secretary Matt Hancock said on Twitter that the findings by the MHRA and EMA had confirmed that the AstraZeneca jab was “safe” and that the benefits of the jab “far outweigh the risks for the vast majority of adults”.