By April 13, 8 cases of thrombotic events associated with thrombocytopenia had been detected among 7,000,000 doses of vaccine administered in the United States. All cases occurred in persons younger than 60 years of age within three weeks of vaccination, mostly in women. Based on currently available evidence, no specific risk factors associated with these blood clotting events have been detected. The European Medicines Agency scientific evaluation continues to support the use of the vaccine, based on its positive risk-benefit ratio. One plausible explanation for the combination of blood clots and low platelets is an abnormal immune response, leading to a condition similar to that sometimes seen in patients treated with heparin called heparin-induced thrombocytopenia, also similar to the one described after vaccination with Vaxzevria (AstraZeneca) vaccine.
Thrombosis (formation of blood clots in the blood vessels) in combination with thrombocytopenia (low levels of blood platelets) following vaccination with Janssen COVID-19 vaccine is possible but very rare.