The European Medicines Agency (EMA) said on Wednesday it had found a possible link between AstraZeneca's coronavirus vaccine and reports of very rare cases of blood clots in people who had received the shot.
Safety concerns have prompted more than a dozen countries in recent weeks to suspend use of the vaccine, which has been given to tens of millions of people in Europe, after reports linking it to a brain blood clotting disorder in a few dozen recipients.
EU health ministers have been told that the EMA's decision on the safety of the vaccine will have immediate implications for vaccination plans and to require a coordinated response, a letter seen by Reuters showed.
The letter was sent by the Portuguese presidency of the EU on Tuesday to invite health ministers for an extraordinary virtual meeting on Wednesday, to be held immediately after the decision by the EU drugs regulator.
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The EMA announcement is a fresh setback for the vaccine, which is sold at cost, for a few dollars a dose, and is by far the cheapest and most high-volume launched so far. After extensive use in Britain and mainland Europe, it is set to be the mainstay of vaccination programmes 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.
Both the EMA and the World Health Organization have said the benefits outweigh the risks of the vaccine.
"The risk of mortality from Covid-19 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.
Last month, AstraZeneca published early results of its US late-stage clinical trial showing the shot was 79 per cent effective, but had to scramble to release more data after a rare rebuke from U.S. health officials, who said the data was outdated.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.