The EU's drugs regulator has said that blood clots should be listed as a "very rare" side effect of the Johnson & Johnson Covid-19 vaccine.
The European Medicines Agency said in a statement that it had found a "possible link" between the jab and clots.
But it added that the benefits of the Covid-19 vaccine outweighed the risks.
Out of more than seven million people who have received the jab in the US, eight people developed rare blood clots, including one person who died.
Earlier this month, the EMA made the same recommendation for the Covid-19 vaccine produced by Oxford-AstraZeneca.
That recommendation, made on 7 April, came after 86 out of 25 million Europeans administered with the jab developed the unusual blood clots.
The UK then stopped administering the AstraZeneca vaccine to people under 30 years old.
Last week, the EU, the US and South Africa paused the rollout of the Johnson & Johnson jab in response to the reports of rare blood clotting.
What did the EMA say?
In a statement released on Tuesday, the EMA recommended that blood clots should be listed as a "very rare" side effect of the Johnson & Johnson vaccine (also known as the Janssen vaccine).
The cases of clotting in the US "occurred in people under 60 years of age within three weeks after vaccination, the majority in women", the regulator said.
"One plausible explanation for the combination of blood clots and low blood platelets is an immune response, leading to a condition similar to one seen sometimes in patients treated with heparin called heparin induced thrombocytopenia, HIT," it said.
Heparin is a blood thinner given to people with clotting disorders.
But the risks associated with the virus itself are still higher than the vaccine, it added.
"Covid-19 is associated with a risk of hospitalisation and death," the EMA said. "The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of [the Johnson & Johnson] Covid-19 vaccine in preventing Covid-19 outweigh the risks of side effects."
These possible side effects seen with the Oxford/AstraZeneca Covid vaccine and now the Janssen/Johnson & Johnson shot are very rare.
It doesn't mean either are no good or unsafe to use in vaccination programmes.
Experts say, for most individuals, the benefits of getting immunised clearly outweigh any potential risks.
Covid infection can be extremely dangerous and cause harmful blood clots at a much higher frequency than these ultra-unusual events that have been linked to vaccination.
Any approved treatment or vaccine will have some side effects and these are listed on or in the pack.
The EMA's safety committee says a warning about rare blood clots should be added to the Janssen vaccine's product information.
Experts are still exploring the link but say it might be related to an unusual immune response in some individuals.
What are the symptoms?
While the risk of developing a blood clot is extremely low, and the risks associated with Covid-19 itself are still greater, the EMA is warning people to be aware of the symptoms of clots if they're getting the Johnson & Johnson vaccine.
The symptoms to be aware of after getting the vaccine are:
Shortness of breath
Persistent abdominal pain
Neurological symptoms, such as severe and persistent headaches or blurred vision
Tiny blood spots under the skin beyond the site of the injection
If a person develops any of these symptoms within three weeks of getting the jab, they should seek urgent medical attention.