Very rare blood clots found in patients who tend to be women younger than 60 may have a “possible link” to AstraZeneca’s COVID vaccine, the European Union’s drug regulator said Wednesday.

The European Medicines Agency said the vaccine “may trigger an immune response” that leads to the clots, but it’s still too early to say for certain.

The regulator declined to impose any new age restrictions on the vaccine’s rollout, stressing “the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects.”

In the U.K., meanwhile, regulators said people younger than 30 should be given the choice of another product.

“The reported cases of unusual blood clotting following vaccination with the AstraZeneca vaccine should be listed as possible side effects of the vaccine,” EMA Executive Director Emer Cooke said Wednesday.

“The risk of mortality from COVID is much greater than the risk of mortality from these side effects,” Cooke said.

The EMA reached its conclusion after studying 62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis reported as of March 22 in Europe and the U.K., where roughly 25 million people have received the AstraZeneca vaccine.

The agency said most of the cases occurred in women younger than 60 within two weeks of vaccination.

Healthcare professionals should tell people receiving the AstraZeneca vaccine that they should seek medical attention if they develop certain symptoms of clots, the EMA said.

The symptoms include shortness of breath, chest pain, leg swelling, persistent abdominal pain, persistent headaches and blurred vision, the EMA said.

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