Do COVID-19 vaccines cause blood clots?

How does the risk of a blood clot from a COVID-19 vaccine compare to the risk from a COVID-19 infection?

For a brief overview on the different types of COVID-19 vaccines and how they work, check out this article!

A serious pause for concern occurred in the pandemic’s mass vaccination when some individuals developed a blood clotting syndrome after having received a vector COVID-19 vaccine, AstraZeneca or Johnson and Johnson. The syndrome is called vaccine-induced thrombotic thrombocytopenia (VITT).

Although more research is needed on the topic, one of the leading explanations as to why VITT happens is that the negative charge of the vector attracts the positive charge of a particular protein found in the blood called platelet factor 4. Occasionally, the immune system confuses this protein as the virus and causes antibodies to attack it. The antibodies clump together with the protein and cause the formation of blood clots.

It is important to put this information into perspective. This complication, and all vaccine complications, are rare. The Journal of the American College of Cardiology (JACC) suggests that the prevalence of VITT may be close to roughly 3.6 people for every million people who receive the AstraZeneca vaccine and 0.9 people for every million people who receive the Johnson & Johnson vaccine. This syndrome is curable if caught early and preventative measures can be taken. Following the administration of the vaccine, blood tests can be done to monitor blood platelet counts. It is also noteworthy that blood clotting issues are a far more frequent consequence of a severe COVID-19 infection. It is estimated that out of 1 million patients hospitalized for COVID-19, 207 will experience blood clotting issues.

Other non-vector COVID-19 vaccines, like mRNA vaccines (Pfizer and Moderna), use a different mechanism of action and have not been associated with blood clotting problems.

References

• Sharifian-Dorche M, Bahmanyar M, Sharifian-Dorche A, Mohammadi P, Nomovi M, Mowla A. Vaccine-induced immune thrombotic thrombocytopenia and cerebral venous sinus thrombosis post COVID-19 vaccination; a systematic review. J Neurol Sci [Internet]. 2021 Sep 15 [cited 2022 Jul 19];428:117607. DOI: 10.1016/j.jns.2021.117607.
• Baker AT, Boyd RJ, Sarkar D, Teijeira-Crespo A, Chan CK, Bates E, et al. ChAdOx1 interacts with CAR and PF4 with implications for thrombosis with thrombocytopenia syndrome. Sci Adv [Internet]. 2021 Dec 3 [cited 2022 Jul 19];7(49):eabl8213. DOI: 10.1126/sciadv.abl8213.
• American College of Cardiology (ACC). Vaccine-induced Thrombotic Thrombocytopenia (VITT) and COVID-19 Vaccines: What Cardiovascular Clinicians Need to Know [Internet]. Washington, DC: ACC; 2021 Jun 8 [cited 2022 Jul 19]. Available from: https://www.acc.org/latest-in-cardiology/articles/2021/04/01/01/42/vaccine-induced-thrombotic-thrombocytopenia-vitt-and-covid-19-vaccines