The European Medicines Agency (EMA), the EU's regulator for new medicines, has launched a review into potential links between the Johnson & Johnson coronavirus vaccine and blood clots.

It follows reports of four serious cases of "unusual blood clots" postvaccination with COVID-19 vaccine Janssen, the EMA said on Friday, using the name of Johnson & Johnson's European subsidiary.

"It is currently not clear whether there is a causal association between vaccination with COVID-19 Vaccine Janssen and these conditions," the EMA said.

Earlier this week, the EMA announced that blood clotting should be listed as a "very rare" side effect of the AstraZeneca-produced shot, following similar blood clot reports. It said the benefits of the vaccine still outweighed any risks.

What is the EMA reviewing?

A special EMA committee is looking into reports of "thromboembolic events." This means the formation of blood clots, resulting in the obstruction of a vessel, in people who received the Johnson & Johnson vaccine.

The blood clots were with low blood platelets — a feature similar to the AstraZeneca blood clot cases. One such reported case was fatal.

What happens next?

The EMA committee will decide whether regulatory action may be necessary. This would likely mean an update to the product information, such as with AstraZeneca. 

The Johnson & Johnson vaccine is currently only being used in the USA, after gaining emergency authorization there.

The EU authorized the vaccine in March 2021, but EU member states have not started using it yet. They are expected to begin vaccine rollouts with the Johnson & Johnson shot in the next few weeks.

kmm/msh (Reuters, AFP, dpa)