Britain's vaccine advisory committee Wednesday said adults under 30 should be offered an alternative to AstraZeneca's COVID-19 vaccine when possible.
That's due to a, quote, "vanishingly rare" side effect of blood clots in the brain.
Chair of Britain's Joint Committee on Vaccination and Immunisation, Wei Shen Lim.
"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."
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.
Now, rising cases caused by more infectious variants are threatening to overwhelm the hospitals of many EU countries - where the pace of vaccinations lags far behind Britain and the United States - and forcing France and others to reimpose social and economic lockdowns.
The European Medicines Agency said it received reports of 169 cases of the rare brain blood clot by early April, after 34 million doses had been administered, which makes it appear to be extremely rare.
In comparison, four women out of 10,000 would get a blood clot from taking oral contraception.
Amid the news about AstraZeneca, British Prime Minister Boris Johnson said he did not expect to change the government's plans for relaxing COVID-19 restrictions.
"I don't think that anything I have seen leads me to suppose that we will have to change the roadmap or deviate from the roadmap in any way."
Britain's vaccine advisory committee on Wednesday said that people should continue to have a second dose of the AstraZeneca shot if they had already received the first dose.