The European Union's drug regulator has identified a possible link between rare cases of blood clotting in deep veins with Johnson & Johnson's (J&J) or Janssen Covid-19 vaccine and recommended the condition be listed as a side-effect of the shot.
The European Medicines Agency (EMA) 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.
The drugmaker said while the chances of experiencing these conditions were very low, the product information would be updated accordingly as it continues to work closely with authorities including EMA.
"We strongly support raising awareness of the signs and symptoms of rare events to ensure they can be quickly identified and effectively treated," the company said.
Both the J&J and AstraZeneca 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 Covid infections.
The 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.