European and British regulators have found possible links between AstraZeneca's vaccine and very rare cases of blood clots, but reaffirmed its importance in protecting people against Covid-19.
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 at one time suspended use of the vaccine, which has been given to tens of millions in Europe. But most have resumed and some, including France, the Netherlands and Germany, have set a minimum age.
Now, rising infections 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 (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. The EEA comprises the 27 EU countries plus Iceland, Norway and Liechtenstein.
In comparison, four women out of 10,000 would get a blood clot from taking oral contraception.
In its statement, the EMA said it was reminding health professionals and recipients 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".
No new guidelines
"So far, most of the cases reported have occurred in women under 60 years of age within 2 weeks of vaccination," it added. But it did not issue any new guidelines.
Professor Frederic Adnet, head of emergency services at Avicenne Hospital in Bobigny, France, said the statement would nevertheless affect take-up in France, where scepticism about vaccines is high.
"The EMA's communication today will undoubtedly affect confidence further in AstraZeneca's vaccine," he said.
EU health ministers began meeting soon after the EMA's statement.
AstraZeneca's shot is sold at cost, for a few dollars a dose. It 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 programmes in much of the developing world.
The World Health Organisation's advisory vaccine safety panel said a causal link to blood clots with low platelets "is considered plausible but is not confirmed".
Experts say that, even if a causal link 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.
Benefits outweigh risks - UK regulator
Nevertheless, AstraZeneca's shares were down 1.2 percent at a two-week low.
The shot has faced questions since late last year, when the drugmaker and Oxford University published trial data with two different efficacy readings as a result of a dosing error.
The firm last month published early results of its US late-stage clinical trial showing the shot was 79 percent effective, but then had to scramble to release more data after a rare rebuke from US health officials, who said the data was outdated.
The head of Britain's medicines regulator, June Raine, said the benefits outweighed the risks for the vast majority, but were more 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.
AstraZeneca has said its own studies have found no higher risk of blood clots in those vaccinated than in the general population.
Scientists are exploring several possible causes for the rare cerebral sinous vein clots. One theory suggests the vaccine triggers an unusual antibody in rare cases; other investigators are looking into a possible link with birth control pills.
But there is no definitive evidence as yet, and many experts say 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 virus.