Britain should not give Oxford/AstraZeneca's COVID-19 vaccine to under 30s where possible, Britain's Joint Committee on Vaccination and Immunisation (JCVI) said on Wednesday, due to a very rare side effect of blood clots in the brain.

Wei Shen Lim, COVID-19 Chair for JCVI, said that based on the available data and evidence, the committee has advised that it is 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 not advising a stop to any vaccination for any individual in any age group. 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.

It came after Britain's MHRA medicine regulator identified a possible side-effect from the COVID-19 vaccine developed by Oxford University and AstraZeneca involving rare brain blood clotting. Chief executive June Raine said that the benefits of the shot outweighed the risks for the vast majority, echoing an update from Europe's medicine regulator also made on Wednesday.

Europe's and Britain's medicine regulators have both previously said that there is no increased risk of blood clots in general from the shot developed by Oxford and AstraZeneca. However, both have been investigating small numbers of reports of a brain blood clots, know as cerebral venous sinus thrombosis (CVST), that have occurred in combination with unusually low blood platelet levels after people have been given the shot.

Deputy Chief Medical Officer Jonathan Van-Tam said the move would have only a negligible impact on the pace of Britain's vaccine rollout. The rollout of Moderna's shot began on Wednesday, while Britain is also deploying Pfizer's vaccine.