All instances of the blood clots occurred in people under 60 years of age.
The European Medicines Agency has said that the benefits of the Johnson & Johnson Covid-19 vaccine outweigh the risks after a small number of rare blood clots were reported.
Use of the company's one-dose vaccine was temporarily halted by US regulators last week.
Now, the EMA has said that its safety committee concluded that a warning about unusual blood clots with low blood platelets should be added to the vaccine's labels.
It said that it had taken into account all the currently available evidence, including eight reports from the US of serious cases of unusual blood clots, where one had a fatal outcome.
All cases occurred in people under 60 years of age within three weeks after vaccination, the majority in women. Based on the currently available evidence, specific risk factors have not been confirmed.
They also noted that the blood clots occurred mostly at unusual sites such as in veins in the brain, and the abdomen and arteries, together with low levels of blood platelets and sometimes bleeding. The cases reviewed were very similar to the cases that occurred with the Covid-19 vaccine developed by AstraZeneca.
"Healthcare professionals and people who will receive the vaccine should be aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within three weeks of vaccination," they said in a statement on Tuesday.
"Covid-19 is associated with a risk of hospitalisation and death. The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of Covid-19 Vaccine Janssen in preventing Covid-19 outweigh the risks of side effects."
