The European Union's drug regulator on Friday identified a possible link between rare cases of blood clotting in deep veins with Johnson & Johnson's COVID-19 vaccine and recommended the condition be listed as a side-effect of the shot.
The European Medicines Agency also recommended that immune thrombocytopenia (ITP), a bleeding disorder caused by the body mistakenly attacking platelets, be added as an adverse reaction with an unknown frequency to the J&J vaccine product information and to AstraZeneca's vaccine.
J&J did not immediately respond to a request for comment.
Both vaccines have previously been associated with a very rare combination of blood clotting and low platelet counts known as thrombosis with thrombocytopenia syndrome (TTS).
The two products are based on harmless vector viruses that instruct human cells to make a protein that primes the immune system against future coronavirus infections.
EMA said the new, possibly life-threatening clotting condition known as venous thromboembolism (VTE) to be included on the J&J product label was separate from TTS.
VTE typically begins by a clot forming in a vein of a leg, arm or groin, which then travels to the lungs and blocks the blood supply there.
Regardless of any vaccine use, VTE is most commonly caused by injury or lack of movement in bedridden patients. Birth control pills and a number of chronic conditions are also seen as risk factors.