Vaccine-induced Thrombotic Thrombocytopenia

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Vaccine-induced Thrombotic Thrombocytopenia (VITT) or Thrombosis with Thrombocytopenia Syndrome (TTS) is a very rare, but serious condition that received a lot of news and social media attention in the context of vaccines for COVID-19. The newly recognized syndrome is different from other types of blood clotting conditions in that it is triggered by the immune system’s response to the COVID-19 vaccine, most commonly ChAdOx1 nCoV-19 (AstraZeneca) and Ad26.COV2.S (Johnson & Johnson). Both these vaccines are using adenoviral vectors (the mRNA vaccines from Moderna and Pfizer, don’t use that vector). Pathologically it is very similar to the autoimmune heparin-induced thrombocytopenia (HIT). VITT is thought to be due to the autoantibodies that are directed against platelet factor 4 which activates platelets and causes thrombosis. The characteristic feature is these blood clots which are often cerebral or abdominal.

VITT appears to occur in 4-6 people per million vaccine doses given. The risk is lower following the second dose. The initial fatality rate was as high as 50% in those who developed it, but most do now recover if it is recognized early and appropriate treatment started. No obvious risk factors have been identified, but it does appear to be more common in those under the age of 50. A previous history of blood clots (such as a deep vein thrombosis) or other non-immune blood disorders are not a risk factor.

The typical symptoms are a sustained and severe headache, abdominal pain, back pain, nausea and vomiting, vision changes, change in mental status, neurologic symptoms/signs, dyspnea, leg pain and swelling, and/or bleeding/petechiae within 4 to 42 days following the administration of the vaccine. Those with those symptoms should have the platelet count and D-dimer measured along with imaging for thrombosis.

The Criteria for Diagnosis:

  • COVID vaccine 4-42 days previously
  • Any venous or arterial thrombosis
  • Thrombocytopenia
  • Positive ELISA for heparin-induced thrombocytopenia

Most are hospitalized for management due to the severity of symptoms and the potentially life-threatening nature of the condition. Initial management is with anticoagulation (usually a non-heparin anticoagulant) and intravenous immune globulin to interrupt the VITT antibody-induced platelet activation. Corticosteroids may be used dampen the abnormal immune response. Refractory disease may need a plasma exchange and further immunosuppression. Daily platelet count monitoring and clinical monitoring for any signs of thrombosis are critical.


  • When reports first emerged early in 2021 on VITT from the AstraZeneca vaccine, regulatory authorities in several countries suspended its use while the risk of the complication was evaluated. They all resumed the use of that vaccine.
  • The social media and news media response to this caution by the regulatory authorities led to significant fear in the community with many wanting the mRNA vaccine rather than one of the adenoviral vector vaccines despite the extremely low risk. Public health responses tried to counter that, trying to put the risk into context (eg you are more likely to be hit by lightning) and try to explain the concept of and meaning of ‘risk’.
  • The community fear related to this was more obvious in countries like Australia who had an extremely low case number of COVID-19 compared to places like the UK where the risks from COVID-19 were much greater. The risk from the vaccine was generally perceived as being much less in the UK.


  • all health professionals have a responsibility to reinforce public health messaging on this.

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