In a statement, the group said that under-30s should be "offered an alternative COVID-19 vaccine, if available."
Most people under 30 have not yet been offered vaccines in the UK, which concentrated on older demographics first.
The reason for the change is a small number of blood clots in people given the vaccine. Officials at the briefing described them as a "suspected" side-effect of the shot, and said that they are very rare.
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They argued that the benefits of the vaccine - preventing people being hospitalized and killed by COVID-19 - still far outweigh the risks.
At the briefing Wednesday, Jonathan Van-Tam, the Deputy Chief Medical Officer for England, described the change as a "course correction."
Dr June Raine, Chief Executive of Medicines and Healthcare products Regulatory Agency (MHRA) said of the blood clots that "the risk of this rare suspected side effect remains extremely small."
The logic of restricting the vaccine from use in younger demographics is that they are less at risk of COVID-19, and slightly more at risk from the clots, officials said at the briefing.
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This chart from the MHRA was shown to describe the balance of risks:
As of March 31, the MHRA recorded 79 cases of the blood clots following the first dose of the AstraZeneca vaccine (51 women and 28 men age from 18 to 79 years). Of those, 19 died, three of whom were less than 30 years old.
The 19 deaths were from many millions of people in the UK to be given the AstraZeneca jab. The country has given out more than 20 million doses. More details on the people who got clots was published in an MHRA press release.
Similar reports of blood clots prompted earlier suspensions of the AstraZeneca rollout in many countries, some of which later restricted the use of the shot in younger age groups.
Countries including, Canada, Germany, France, and Italy have suspended the use of the vaccine in younger populations pending the results of the investigation.
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The shot has not been approved in the US. Dr. Antony Fauci, the White House chief medical advisor the US may end up not needing the AstraZeneca vaccine for its vaccine rollout.
It also makes up a substantial proportion of vaccination programmes in other countries, including lower income countries depending on the COVAX scheme.
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