The European Medicines Agency (EMA) says it has launched a review into a possible link between blood clots and the Johnson & Johnson vaccine.
It comes after four cases of serious blood clots were reported, one of them deadly.
Three of these cases were reported in the United States after authorisation of the vaccine. One case was reported during clinical trials in an unspecified country.
The EMA said its safety committee was "starting a review" that would "assess reports of thromboembolic events (formation of blood clots, resulting in the obstruction of a vessel) in people who received" the Johnson & Johnson vaccine.
The European Union authorised the jab in March but the vaccine will not be delivered to EU countries until April 19.
It comes after the EMA said earlier this week that there was a possible link between the AstraZeneca jab and "very rare cases" of blood clots.
The regulator was at pains to point out, however, that the benefits of the vaccine outweigh the risks of side effects, explaining that COVID-19 is a "very serious disease".