What is the incidence of thromboembolic events (blood clots) associated with Coronavirus disease 2019 (COVID-19) vaccines?

From the Guidelines

Blood clots were a rare side effect of certain COVID-19 vaccines, occurring in approximately 3-4 cases per million doses of adenoviral vector-based vaccines. The risk of blood clots from COVID-19 infection itself is significantly higher, with studies showing a greater risk of clots from the disease than from vaccination 1. The vaccines associated with this rare condition, called thrombosis with thrombocytopenia syndrome (TTS) or vaccine-induced immune thrombotic thrombocytopenia (VITT), were primarily adenoviral vector-based vaccines. Some key points to consider include:

• The diagnosis of VITT can be established based on a high degree of suspicion, defined as a high D-dimer and thrombocytopenia (<150 000/μL) and thrombosis occurring 5-30 days after vaccination with adenoviral vector-based vaccine for COVID-19 1.
• Patients with a history of thromboembolism have been naturally concerned about the risk of recurrent thrombosis after vaccination, but a large observational study in patients with a diagnosed thrombophilia defect did not show increased risk for thrombosis during 3 months after almost exclusive use of the messenger RNA vaccines 1.
• The treatment of VITT is complex, which makes the interpretation of studies that focus on a single agent challenging 1.
• The benefits of COVID-19 vaccination in preventing severe disease and death have consistently outweighed these rare risks. Some important considerations for patients and healthcare providers include:
• Being aware of the symptoms of blood clots, such as severe headache, abdominal pain, leg pain, or shortness of breath.
• Monitoring for these symptoms, particularly in women under 50 years old, within 1-2 weeks after vaccination.
• Weighing the rare risks of blood clots against the significant benefits of COVID-19 vaccination in preventing severe disease and death.

From the Research

Covid Vaccine-Induced Blood Clots

• The COVID-19 vaccines have been associated with rare cases of blood clots, including cerebral venous sinus thrombosis (CVST) and splanchnic venous thrombosis 2, 3, 4, 5, 6.
• The incidence of vaccine-induced immune thrombotic thrombocytopenia (VITT) is estimated to be around 9-31 events per one million doses of adenovirus-based vaccines 3.
• VITT is more common in females and young vaccine recipients, with a higher incidence in those under 60 years of age 2, 3, 5.
• Adenovirus-based vaccines have been associated with a higher risk of thrombosis without thrombocytopenia (non-VITT thrombosis) compared to mRNA-based vaccines 3.
• The incidence of non-VITT thrombosis is 5-10 fold higher than that of VITT, and 7-12 fold higher than observed in recipients of mRNA-based vaccines 3.

Risk Factors and Treatment

• Female sex and age younger than 60 years have been identified as possible risk factors for VITT 2, 5.
• Treatment for VITT consists of therapeutic anticoagulation with nonheparin anticoagulants and prevention of formation of autoantibody-PF4 complexes 2, 4.
• High-dose intravenous immunoglobin (IVIG) and steroids may also be used in combination with anticoagulation therapy 2, 4.
• Plasma exchange and monoclonal antibodies, such as rituximab and eculizumab, may be considered in severe cases 2, 4.