The European Union’s drug regulator 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 immunization with the J&J shot, the European Medicines Agency said Friday. The regulator is now scrutinizing potential safety issues for two COVID vaccines, after AstraZeneca Plc’s shot was possibly linked to a rare blood-clotting disorder.

J&J is working with regulators to assess data on the rare clots, and “at present, no clear causal relationship has been established” with the vaccine, the company said in an e-mailed statement. The shares traded as much as 1.6 per cent lower.

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 needs two doses.

Sputnik Data

The EU regulator also started a review to assess five reported cases of a rare disorder called capillary leak syndrome in people who got the AstraZeneca shot. The EMA said it’s not clear whether the vaccine was linked to the disorder, in which fluid leaking from blood vessels causes tissue to swell and blood pressure to drop.

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

The J&J, Astra and Sputnik shots all use an adenovirus -- the cause of some common colds -- to deliver the coronavirus antigen and generate an immune response.

Adenovirus technologies such as that used by AstraZeneca and others have been associated with clotting in other settings, so if this is the reason for the rare side effects observed with the Astra vaccine, shots from J&J, Sputnik and Chinese drugmaker CanSino Biologics Inc. would also be at risk, said Sam Fazeli, an analyst with Bloomberg Intelligence.

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

Clotting Cases

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.

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

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.

--With assistance from Riley Griffin and Todd Gillespie.