Johnson & Johnson said it’s delaying the rollout of its COVID-19 vaccine in Europe after U.S. regulators paused immunizations with the shot pending a review of rare blood clots.

The company is reviewing the cases with European health authorities, Johnson & Johnson said in a statement Tuesday. U.S. officials suspended vaccinations after six women suffered a type of brain blood clot similar to that reported as a rare side effect to the AstraZeneca Plc vaccine. About 6.8 million people in the U.S. have received the J&J vaccine.

The incident is another blow to efforts to vaccinate the world and bring the pandemic to a close. The European Union was relying on the single-dose vaccine to boost its immunization drive, and deliveries to the 27 member states had just started this week.

The bloc had been expecting some 55 million doses of the J&J vaccine this quarter as it ramps up vaccination efforts. While the shot is approved in the EU, the inoculation campaign hasn’t started.

The EU’s drug regulator last week started a review to assess reports of clots in people who received the J&J vaccine. The vaccine is under review in the U.K. but hasn’t been approved there yet.

The J&J vaccine, like the one developed by Astra and the University of Oxford, uses an adenovirus to deliver the genetic material into the body to provoke a defense against COVID-19. The Oxford-Astra vaccine uses a chimpanzee adenovirus to achieve the response, while J&J’s is derived from humans. Russia’s Sputnik V vaccine and one from China’s CanSino Biologics Inc. also rely on that approach.