The scientific committee overseeing Britain's coronavirus vaccination programme on Friday recommended that under-40s are offered an alternative to the Oxford-AstraZeneca Covid jab.

The Joint Committee on Vaccination and Immunisation (JCVI) said it was taking the "precautionary approach" for adults aged 30-39, after assessment of blood clot risks.

Professor Wei Shen Lim of the JCVI advisory committee said that those aged 30-39 will be "preferentially offered an alternative to the AstraZeneca vaccine".

This will happen as long as an alternative is available and does not create a substantial delay in vaccination, and as long as the UK keeps its virus situation under control, he added.

Lim said the aim was to "further increase vaccine confidence" as under-40s are due to be vaccinated soon, by showing that the government is putting a "high priority on safety".

The UK -- which launched its mass vaccination drive in December last year with the Pfizer-BioNTech shot -- is still on track to give all adults a first vaccine dose by the end of July, Lim said.

He added that the success of the vaccine rollout means that "a future wave of infection is likely to be smaller than anticipated".

June Raine, who heads the UK's Medicines and Healthcare products Regulatory Agency, said it was not changing its advice on the AstraZeneca vaccine as side effects were "extremely rare".

From more than 28 million first doses of the vaccine administered in the UK by April 28, there were 242 cases reported of clots combined with low blood platelet levels, or 10.5 per million, she said.

These clots occurred in 141 women and 100 men aged from 18 to 93, and the overall case death rate was 20 percent, with 49 deaths.

Six cases have been reported after a second dose of the vaccine.

Raine said this meant that the benefits of the AstraZeneca vaccine still outweigh the risks of coronavirus for the vast majority of people.

For younger people, this ratio is "more fully balanced," she said, however.

Nearly 128,000 people have died from the virus in the UK, the highest figure in Europe.

The UK has now administered nearly 35 million first doses of vaccine and more than 16 million second doses.

A government spokesman said it would follow the advice, adding: "The Oxford-AstraZeneca vaccine is safe, effective and has already saved thousands of lives in the UK and around the world...

"More than 50 million vaccines overall have already been administered, and our current vaccine supply and rate of infection means we are able to take this precautionary step while remaining on track to achieve our target of offering a vaccine to all adults by the end of July."