Europe’s drug regulator says it is reviewing rare blood clots in four people in the U.S. who received Johnson & Johnson’s COVID-19 vaccine.

In a statement, the European Medicines Agency (EMA) said its safety committee had started a review to assess reports of thromboembolic events, which occurred through the formation of blood clots and resulted in the obstruction of a vessel in people who received the Janssen jab, Xinhua news agency reported on Friday.

Johnson & Johnson said it was aware of the rare reports of blood clots in individuals given its COVID-19 vaccine and was working with regulators to assess the data and provide relevant information.

“At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine,” the company said.

Of the four serious cases of "unusual blood clots with low blood platelets," three occurred in the United States, which had started the vaccine's rollout, while the fourth occurred during a clinical trial. One person died as a result, the European Medicines Agency (EMA) said.

The single-dose Janssen (Johnson & Johnson) vaccine is currently only administered in the US under an emergency use authorization. The jab was authorized in the European Union in March, but the rollout has not started yet in any EU member state.

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