British officials and ministers sought to shore up confidence in AstraZeneca's COVID-19 vaccine on Thursday, saying advice that most people under 30 should be offered alternative shots was not unusual and would not impact the pace of rollout.

A pharmacist whose brother died from a brain blood clot linked to the AstraZeneca shot was among those calling for people to keep getting it, saying the doses would save lives.

Officials said the suggestion that under-30s should be offered an alternative did not reflect any serious safety concerns, just a "vanishingly" rare possible side effect.

Anthony Harnden, Deputy Chairman of the Joint Committee on Vaccination and Immunisation (JCVI) which issued the new advice, said such suggestions were not unusual, pointing out that people of different ages already got different flu shots in Britain.

"This isn't unusual. So this is not undermining what the regulators are saying. The regulators are saying this vaccine is suitable for all age groups but it's up to the individual countries to decide how best to deploy those vaccines," he told Reuters in an interview.

While Britain's MHRA medicine regulator did not place age restrictions on the use of the AstraZeneca shot, some saw the JCVI's advice about under-30s, made in the same briefing, as mixed messaging.

Conservative lawmaker Iain Duncan Smith said the announcement was "ridiculous" and that there was "the dangerous potential" that people would refuse to have AstraZeneca's shot.

Health minister Matt Hancock defended the move, saying the transparency over possible side effects, even very rare ones, should bolster confidence in the system.

Britain has continued to use its homegrown AstraZeneca vaccine since it became the first country to start rolling it out at the start of January. Some countries in Europe, including France, restricted its use in older people initially, citing a lack of data, and are now limiting its use in older people.

Harnden said Wednesday's announcement had not been taken lightly and it had been right to keep using the shot, even if the advice was now changing.

"Stopping and starting and changing vaccination programmes is not an easy thing to do, and if you do it, it runs the danger of losing confidence in that programme," he said.

Still on track

Britain is leaning on AstraZeneca for a large portion of its vaccine supply, with 100 million doses ordered.

But it has also has been rolling out vaccines made by Pfizer, since December, and Moderna since Wednesday. Shots made by Johnson & Johnson and Novavax are also pending regulatory approval in the coming months.

Hancock said that with 40 million Pfizer and 17 million Moderna vaccines ordered, there was more than enough shots to cover the 8.5 million people under-30s who needed to be vaccinated. Healthy under-30s are in the last priority cohort to be vaccinated, with most not eligible until the summer.

"We feel that we've got enough vaccine supply to be able to offer (an) alternative vaccine, without delaying the progress of our immunisations," JCVI's Harnden said, adding Britain was on track to give a first shot of vaccine to all adults by the end of July.

"It may slip by a week or two, but no more than that," he said.

The advice did not change for under-30s who have underlying conditions and are eligible for shots now, meaning only very few people who are due to have a shot in the coming days will be affected by the advice change.

Alison Astles, a pharmacist whose 59-year-old brother died of a blood clot on the brain on Sunday, encouraged people to still get their AstraZeneca shots even though it was "very strongly suggested" his death was due to the shot.

"The sister in me still feels absolutely furious and very angry that this has happened to my brother... But despite what has happened to Neil, and the impact on our family, I still strongly believe that people should go ahead and have the vaccine," she said.

"Overall, we will save more lives by people having the vaccine than not."