Restriction of the use of the vaccine in patients with risk factors for thrombosis is not indicated at this time.
Very rare (less than one in million vaccinated to date) cases of serious thrombosis associated with thrombocytopenia, sometimes with bleeding and disseminated intravascular coagulation, have been reported including several cases of cerebral venous sinus thrombosis. Most have occurred within 14 days after vaccination. Given the aggregation of cases, the fact that it is a very rare entity and a plausible temporal relationship, EMA has established that these rare cases may be a possible adverse reaction to the vaccine and are object of special attention by physicians. The underlying cause and mechanisms involved in such events is yet to be established. However, we do know that COVID-19 disease is associated, by itself, with an increase in thrombotic events and that such events have been documented in patients with asymptomatic COVID-19 disease; we also have seen similar conditions would be triggered by an immune response against platelets in heparin-induced thrombocytopenia, resulting in aggregation, thrombosis, and platelet penia. So again, even if this thrombocytopenic thrombotic phenomenon were of an immune nature, restriction of the use of the vaccine in patients with risk factors for thrombosis is not indicated at this time.
A vaccinated person, regardless of the vaccine used, should continue the usual treatment he or she may receive (including any antithrombotic treatment); and no administration of any antithrombotic as a preventive measure, as part of the COVID-19 vaccination, should be considered.
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