All four cases of unusual clots accompanied by low blood platelets have occurred in the U.S. — one in a clinical study and three in recipients after the vaccine gained emergency authorization. One case was fatal.

The vaccine is also authorized in Europe and countries are expected to start rolling it out in the next few weeks, the European Medicines Agency’s safety arm said in a news release. The reports suggest there’s a “safety signal.”

“But it is currently not clear whether there is a causal association between vaccination with Covid-19 Vaccine Janssen and these conditions,” the EMA said.