Under-40s are to be offered an alternative to the Oxford/AstraZeneca vaccine due to a link with extremely rare blood clots.

The UK's medicines regulator, the MHRA, said 242 blood clots had been discovered among 28 million people who received first doses of the AZ jab up to April 28.

Diss Mercury: Over-70s could be given booster jabs in the autumn to protect against new coronavirus variantsOver-70s could be given booster jabs in the autumn to protect against new coronavirus variants (Image: Archant 2021)

That is an incidence rate of 10.5 per million.

Just six clots were reported among the six million who received second doses of the vaccine - one in a million.

The risk has, however, been identified as slightly higher in younger age groups, resulting in a change to the Joint Committee on Vaccination and Immunisation's (JCVI) guidance.

Vaccine-in-numbers--More-than-16-million-receive-second-dose (1)

Recipients aged between 30 and 39 will therefore be given different jabs, such as the Pfizer or Moderna products, in the months ahead.

In comes exactly a month after everyone under the age of 30 was told they would no longer be offered the AZ vaccine.

Professor Wei Shen Lim, chairman of the JCVI, emphasised that evidence was still "very much in favour" of vaccination.

He added: "We now advise that unvaccinated adults aged 30 to 39 years who do not have an underlying health condition should be preferentially offered an alternative to the AZ vaccine".

But Prof Lim highlighted that the alternative offer should occur "only where no substantial delay in access to vaccination might arise".

He said the decision had been made "in the context" of the UK rollout, with “good control” of infection and availability of other vaccines.

Diss Mercury: A Covid jab is administered at Connaught Hall, AttleboroughA Covid jab is administered at Connaught Hall, Attleborough (Image: Archant 2021)

Providing further background to the change, Dr June Raine, chief executive of the MHRA, said there was no change to the agency's approval of the jab.

She added: "The benefits of the AZ vaccine against Covid-19, with the associated risks of hospitalisation and death, continue to outweigh the risks of the vaccine for the vast majority of the people.

“The balance of benefits and risks of the vaccine is very favourable for older age groups, but it is more finely balanced for younger people.

Diss Mercury: The mass Covid vaccination centre at King's Lynn Corn ExchangeThe mass Covid vaccination centre at King's Lynn Corn Exchange (Image: Ian Burt Photography)

"And so, we advise that this evidence should be taken into account when considering the use of the vaccine."

Meanwhile, Jonathan Van Tam, deputy chief medical officer for England, confirmed the rollout was still on target to offer a first dose to all adults by the end of July.