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So far, the unusual combination of blood clots and low platelets remains extremely rare. In a press conference Wednesday, the EMA said it has identified 169 cases of clotting in the veins that drain blood from the brain as well as 53 cases in abdominal veins — 18 of which were fatal — among 34 million vaccinations with AstraZeneca’s shot given to date in the U.K. and Europe.

Detection of the events is evidence “our pharmacovigilance system is working,” said EMA director Emer Cooke.

Still, the situation has raised important questions about the safety of AstraZeneca’s vaccine at a critical time. Coronavirus infections are rising across Europe, where the shot is one of only a few available and vaccine hesitancy is a concern. Several nations have since restarted use of the shot after an initial wave of suspensions last month, but others have changed recommendations of who should be immunized, potentially eroding public confidence.

Testing has shown AstraZeneca’s vaccine to be strongly protective against COVID-19, a benefit that’s held up through extensive real world use in the U.K.

That’s why Cooke and other EMA representatives, concerned the announcement may sow further doubt in AstraZeneca’s shot, strongly supported use of AstraZeneca’s vaccine at a briefing Wednesday.

“It’s very important that we use the vaccines that we have available to us,” Cooke said, adding that the “risk of mortality from COVID is much greater than the risk of mortality from the side effects.”

A “plausible explanation” exists that the side effects may share similarities with a condition known as heparin-induced thrombocytopenia, said EMA safety chair Sabine Strauss.

But so far, investigators haven’t been able to confirm any specific risk factors, even though the events appear to be most prevalent in women under 60 years old. That finding could simply be a result of how the vaccine has been used in Europe versus the U.K., said Cooke. Many countries had previously limited immunizations in the elderly because AstraZeneca didn’t have definitive proof, until recently, that shots would be equally protective.

Strauss said AstraZeneca will be required to do additional studies to investigate the reactions while vaccinations continue. The U.K.’s drug regulator on Wednesday similarly urged support of AstraZeneca’s shot, calling the side effects “extremely rare.”

A U.K. committee in charge of issuing guidance on vaccine use, however, did recommend people under the age of 30 without underlying health conditions be given another vaccine over AstraZeneca’s.

The EMA was also asked Wednesday whether it was concerned the side effect might also occur with Johnson & Johnson’s shot as well. Though there are important differences between the two vaccines, they rely on the same type of technology. So far, just three cases of abnormal blood clots with low platelets have been reported with J&J’s shot, said Peter Arlett, the EMA’s head of data analytics and methodology. “The situation, however, is under close scrutiny,” he said.

The findings come as AstraZeneca is preparing to ask for emergency authorization in the U.S. That evaluation could be contentious given AstraZeneca’s unusual public dispute with a data review board and the lengthy pause last fall of the Phase 3 study supporting its application.

AstraZeneca said it didn’t see any evidence of increased risk of blood clots in the trial, which enrolled a little more than 32,000 people.