* UK says 18-29 year olds should get other shots where possible

* Oxford/AZ shot associated with rare cases of brain blood clots

* Benefits outweigh the risks for vast majority, regulator says

(Adds PM Johnson&aposs reaction)

By Guy Faulconbridge, Kate Holton and Alistair Smout

LONDON, April 7 (Reuters) - Britain should give an alternative to Oxford/AstraZeneca&aposs AZN.L COVID-19 vaccine to under 30s where possible due to a "vanishingly" rare side effect of blood clots in the brain, Britain&aposs vaccine advisory committee said on Wednesday.

Officials said the benefits of the shot continued to outweigh its risks for the vast majority, after safety concerns spurred over a dozen countries in recent weeks to suspend use of the vaccine following reports linking it to a brain blood clotting disorder. Britain has kept using the shot throughout.

Wei Shen Lim, chair of Britain&aposs Joint Committee on Vaccination and Immunisation (JCVI), said that based on the available data and evidence, it was preferable for adults aged under 30 with no underlying conditions to be offered an alternative to the AstraZeneca vaccine where available.

He said that for younger people, where the risks of hospitalisation were much lower, the risk/benefit calculation of the Oxford/AstraZeneca shot meant others vaccines were preferable.

"We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution, rather than because we have any serious safety concerns," Lim said at a briefing.

He said people should continue to have a second dose of the AstraZeneca shot if they had received a first dose.

June Raine, chief executive of the UK&aposs MHRA medicine regulator, said that over 20 million AstraZeneca vaccine doses had been given by March 31, with 79 case reports of the side effect and 19 deaths, all after first doses of the shot.

"This is a vanishingly rare, but sadly quite serious, adverse event..., and you can&apost pick these kind of things up until you have literally deployed tens of millions of doses of vaccine," Deputy Chief Medical Officer Jonathan Van-Tam said.

He added that the move would have only a negligible impact on the pace of Britain&aposs vaccine rollout.

The rollout of Moderna&aposs MRNA.O shot began on Wednesday, while Britain is also deploying Pfizer&aposs PFE.N vaccine, and Prime Minister Boris Johnson said the move on AstraZeneca would not impact England&aposs roadmap out of lockdown.

"As the regulators have said, this vaccine is safe, effective and has already saved thousands of lives - and the vast majority of people should continue to take it when offered," he said.

GETTING FIRMER

The change to advice on Britain&aposs vaccine rollout came after its MHRA medicine regulator said the reports of rare brain blood clotting were a suspected side-effect from the COVID-19 vaccine developed by Oxford University and AstraZeneca.

MHRA chief Raine said the benefits of the shot outweighed the risks for the vast majority, echoing an update from Europe&aposs medicine regulator also made on Wednesday.

Europe and Britain&aposs medicine regulators have been investigating small numbers of reports of brain blood clots, known as cerebral venous sinus thrombosis (CVST), that have occurred in combination with unusually low blood platelet levels after people have been given the shot.

Munir Pirmohamed, chair of the Commission on Human Medicines, said the link between cerebral blood clots, lower platelets and the AstraZeneca vaccine was "getting firmer".

"Early evidence suggests that this constellation of symptoms is caused by an immune response against platelets which allows the platelets to then lead to clotting in different parts of the body," he told a news conference.

"But what we don&apost have clearly is the link between the vaccine and how the immune response becomes activated against the platelets, and that&aposs where the scientific work needs to go on to identify what that link is."