One person has died from a blood clot after receiving the Johnson & Johnson vaccine.
The European Medicines Agency (EMA) has launched a review to assess reports of blood clots in people who have received Johnson & Johnson's Covid-19 vaccine.
Four serious cases of unusual blood clots with low blood platelets have been reported after people received the Johnson & Johnson vaccine.
One case occurred in a clinical trial and three occurred during the vaccine rollout in the USA, one of which was fatal.
The vaccine is currently only used in the USA, under an emergency use authorisation.
It was authorised in the EU
last month, but the rollout of the vaccine has not started yet in any EU member states.
It is expected to begin in the next few weeks, with Irish Health Minister Stephen Donnelly revealing
earlier this week that just over 40,000 of the vaccines are expected to arrive in Ireland this month.
The EMA's statement read: "It is currently not clear whether there is a causal association between vaccination with Covid-19 Vaccine Janssen and these conditions."
Janssen Pharmaceutica is a Belgium-based division of Johnson & Johnson.
The regulator says its safety committee is now investigating these cases and will decide whether regulatory action may be necessary. This usually consists of an update to the product information.
The EMA has said it will provide further information once the evaluation has concluded.
On Wednesday, the EMA concluded that unusual blood clots should be listed as a "very rare" side effect of AstraZeneca's Covid-19 vaccine, while stating that the benefits of the vaccine continue to outweigh the risks for people who receive it.
Meanwhile, the Health Products Regulatory Authority (HPRA) has confirmed it is investigating
the first case in Ireland of a very rare blood clot in a person after receiving the AstraZeneca Covid-19 vaccine.
