The European Union’s drug regulator said it has started a review to assess blood clots in people who received Johnson & Johnson’s COVID-19 vaccine.

Four serious cases of unusual clots accompanied by low blood platelets, one of which was fatal, have emerged after vaccination with the J&J shot, the European Medicines Agency said Friday. The move turns the regulator’s safety scrutiny on a second COVID shot, after AstraZeneca Plc’s vaccine was possibly linked to a rare blood-clotting disorder.

One of the cases following J&J vaccinations happened during a clinical trial. At the time, the company said it had found no evidence the vaccine was at fault. Three others occurred in the U.S., where the shot has been given to almost 5 million people.

Though J&J’s vaccine is approved in the EU, its rollout in the bloc isn’t expected to start until later this month. Still, the EU is relying on the one-shot vaccine to boost its immunization drive amid restrictions in some countries on use of the AstraZeneca shot, which is delivered in two doses.

Separately, the regulator said it doesn’t yet have enough evidence to approve the Russian Sputnik V vaccine.

J&J representatives didn’t immediately respond to an email seeking comment. The drugmaker’s vaccine uses similar technology to both the AstraZeneca shot as well as Sputnik.

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The Food and Drug Administration didn’t immediately respond to a request for comment.

The numbers of clot cases in people who took the J&J shot have been small so far compared to the total number of people getting the vaccine worldwide, Peter Arlett, EMA’s head of analytics, said on April 7. At that point, Arlett said three clotting cases had been found, while some 4.5 million people had gotten the J&J shot.

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“I think it would be fair to say there is intensive monitoring of this issue across the vaccines,” Arlett said.

Like the J&J and Astra shots, Sputnik uses a chimpanzee adenovirus — the cause of some common colds — to deliver the coronavirus antigen and generate an immune response.

The EMA will continue its rolling review of the Russian vaccine until it has enough evidence to support an application for marketing authorization, the agency said on Friday. The EMA declined to comment on safety information for Sputnik before its assessment is complete.