People under 30 in the UK should not be given the Oxford/AstraZeneca coronavirus vaccine due to "extremely rare" blood clot side effects.
The Joint Committee on Vaccination and Immunisation (JCVI) said under-30s should instead be offered the Pfizer or Moderna jabs.
It follows an investigation by the Medicines and Healthcare products Regulatory Agency (MHRA) into whether the jab is directly causing rare brain blood clots.
The MHRA said that while the benefits of the Oxford/AstraZeneca vaccine continue to outweigh any risks for "the vast majority of people", there have been an "extremely small" number of blood clots.
According to the most recent MHRA data as of 31 March, there were 79 blood-clotting cases, alongside low levels of platelets, among more than 20 million Oxford/AstraZeneca doses administered. Of these, 19 people have died, 11 of whom were under 50 and three under 30.
Speaking at a televised briefing on Wednesday, Dr June Raine, chief executive of the MHRA, made a point of reiterating: "This is extremely rare."
Dr Raine also said "more work is needed to establish beyond all doubt that the vaccine has caused these side effects".
Prof Wei Shen Lim, chair of the JCVI, said the recommendation to offer alternative vaccines to under-30s was “out of the utmost caution” rather than because of “any serious safety concerns”.
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Shortly before the briefing, a review by the European Medicines Agency’s (EMA) safety committee concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the Oxford/AstraZeneca jab.
However, it also said the "overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects".
Some European countries have already restricted the vaccine's use in younger people.
Leading Wednesday's briefing at the Department of Health, England's deputy chief medical officer Prof Jonathan Van-Tam said the revised policy on the Oxford/AstraZeneca jab was a "course correction" – and insisted it should not be a cause for concern.
He said it was “quite normal” and “business as usual” for medics to alter their preferences on how to treat patients.
“This is a massive beast that we are driving along at enormous pace with enormous success, this vaccine programme.
“If you sail a massive liner across the Atlantic then it’s not really reasonable that you aren’t going to have to make at least one course correction during that voyage.”
Responding to the announcements, Boris Johnson said: “We will follow today’s updated advice, which should allow people of all ages to continue to have full confidence in vaccines, helping us save lives and cautiously return towards normality.”
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