A British government advisory group said the vaccine should not be given to the under 30s where possible, though an official said this was "really out of the utmost caution, rather than because we have any serious safety concerns."
More than a dozen countries have suspended use of the vaccine, which has been given to tens of millions of people in Europe, after reports linking it to blood clotting in a few dozen recipients.
In its statement, the European Medicines Agency (EMA) said it was "reminding healthcare professionals and people receiving the vaccine to remain aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within 2 weeks of vaccination."
"So far, most of the cases reported have occurred in women under 60 years of age within 2 weeks of vaccination," it added. "Based on the currently available evidence, specific risk factors have not been confirmed."
The EMA received reports of 169 cases of the rare brain blood clot by early April, after 34 million doses had been administered in the European Economic Area, according to Sabine Straus, chair of the EMA's safety committee.
In comparison, four women out of 10,000 would get a blood clot from taking oral contraception.
"We know that we are rolling out vaccines on a very large scale. We will see events occurring.… Some of them by chance," Straus said.
EU health ministers were told in a letter from the EMA seen by Reuters that the announcement on the safety of the vaccine would have immediate implications for vaccination plans and require a coordinated response.
The chief executive of Britain's health regulator, June Raine, said the benefits of the vaccine outweighed the risks for the vast majority of people. She said those risks were more finely balanced for younger people – for whom the risks from coronavirus infection are, on average, lower.
Wei Shen Lim, COVID-19 Chair for Britain's advisory Joint Committee on Vaccines and Immunisation, said it was preferable for adults under 30 with no underlying health conditions to be offered another vaccine.
The letter to the EU health ministers was sent by the Portuguese presidency of the EU on Tuesday, April 6, to invite them to an extraordinary virtual meeting on Wednesday, to be held immediately after the decision by the EU drugs regulator.
The AstraZeneca vaccine is sold at cost, for a few dollars a dose, and is by far the cheapest and most high-volume launched so far, and has none of the extreme refrigeration requirements of some other COVID-19 vaccines.
After extensive use in Britain and mainland Europe, it is set to be the mainstay of vaccination programs in much of the developing world.
Experts say that, even if a causal link between the vaccine and blood clots is proved, the risks to the general population of getting a serious clot are vanishingly small compared to the risks from possible COVID-19 infection, which can also cause similar clots, or from many other widely used drugs such as the birth control pill.
"The risk of mortality from COVID is much greater than the risk of mortality from these rare side-effects," EMA's executive director Emer Cooke said on Wednesday.
AstraZeneca has said previously its studies have found no higher risk of blood clots in those vaccinated than in the general population.
But the shot has faced questions since late last year, when the drugmaker and Oxford University published data from an earlier trial with two different efficacy readings as a result of a dosing error.
AstraZeneca last month published early results of its US late-stage clinical trial showing the shot was 79% effective, but then had to scramble to release more data after a rare rebuke from US health officials, who said the data was outdated.
Scientists are exploring several possibilities that might explain the rare brain blood clots. One theory suggests the vaccine triggers an unusual antibody in some rare cases; other investigators are looking into a possible link with birth control pills.
But many experts say there is no definitive evidence, and that it is not clear whether or why AstraZeneca's vaccine would cause a problem not shared by other vaccines that target a similar part of the coronavirus.
While many countries have resumed using the shot, some have imposed age restrictions, with younger women perceived to be more vulnerable. – Rapper.com