Blood clots should be listed as a "very rare" side effect of the Johnson & Johnson coronavirus vaccine but its benefits still outweigh the risks, the EU's drug watchdog said on Tuesday.

The European Medicines Agency (EMA) said in a statement that it had found a "possible link" between the jab and the clots, following eight such cases in the United States, one of which was fatal.

Concerns over the vaccine by US pharmaceutical giant Johnson & Johnson and the jab by rival drugmaker AstraZeneca have dampened hopes that vaccines could offer a swift end to the pandemic.

"EMA finds possible link to very rare cases of unusual blood clots with low blood platelets," the Amsterdam-based agency said in a statement, adding that it "confirms (the) overall benefit-risk remains positive."

The regulator said its safety committee "concluded that a warning about unusual blood clots with low blood platelets should be added to the product information" for the J&J shot.

Its experts also "concluded that these events should be listed as very rare side effects of the vaccine."

"The cases reviewed were very similar to the cases that occurred with the Covid-19 vaccine developed by AstraZeneca, Vaxzevria," the EMA statement said.

The eight cases in the US, out of some seven million people who had received the vaccine, all involved people under the age of 60, the majority of whom were women, it said.

"Based on the currently available evidence, specific risk factors have not been confirmed," the EMA said.

EMA chief Emer Cooke was due to hold a press conference on the decision at 1500 GMT.

Adenovirus technology

Johnson & Johnson last week delayed the rollout of its single-shot jab across Europe pending the result of the EMA probe.

J&J said earlier Tuesday that it was "very confident" in its vaccine and hopeful for a quick resolution from regulators over its status.

The EU approved the Johnson & Johnson shot on March 11 and started taking delivery of the vaccine on April 19.

But with concerns already mounting over clots linked to AstraZeneca, the EMA announced on April 9 that it was also probing cases connected to the J&J vaccine.

US regulators have suspended the Johnson & Johnson vaccine and is set to announce its decision on Friday.

Both the Johnson & Johnson and AstraZeneca vaccines use the same adenovirus vector technology.

They use a common-cold causing adenovirus, modified so it cannot replicate, as a "vector" to shuttle genetic instructions into human cells, telling them to create a protein of the coronavirus, and training them to be ready for live Covid.

AstraZeneca opted for a chimpanzee adenovirus, J&J for a human adenovirus. 

Other Covid adenovirus vector vaccines include Russia's Sputnik V and China's CanSino.

Many European countries have continued to restrict the use of AstraZeneca, despite the EMA declaring it safe and saying blood clots are only a "very rare" side effect.