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What To Know About The Rare Blood Clots Linked To COVID-19 Vaccines, Explained By An Expert


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockApr 21 2021, 11:31 UTC
Image Credit: Studio Romantic/

Image Credit: Studio Romantic/

Recently, there has been a huge focus on rare side effects of some vaccinations against SARS-CoV-2, the virus that causes COVID-19. A small number of people that received vaccines such as Oxford/AstraZeneca and Johnson & Johnson developed rare types of blood clots.

It is important to stress how rare these cases are. When the European Union momentarily paused the administration of the AstraZeneca vaccine, there had only been a few dozen cases of cerebral venous sinus thrombosis (CVST) among 17 million doses administered across the continent.


When the Food & Drug Administration (FDA) and the Center for Disease Control (CDC) paused Johnson & Johnson vaccine, there had been six cases of CVST among seven million people vaccinated with that particular jab.

Both vaccines are based on genetically modified adenoviruses – which don't replicate inside cells or cause disease – but can train the immune system to fight a pathogen (SARS-CoV-2).  

It is also important to state you are thousands of times more likely to die of COVID-19 than getting CVST from the vaccine, and you’re eight times more likely to get CVST from COVID-19 than from the vaccine.
This is the crucial context for the important discussion on COVID-19, vaccines, and blood clot risks. The mechanism behind why the vaccine could cause these is still not completely understood, but researchers now have a better picture of symptoms to look out for, as well as therapies.

They are referring to it as vaccine-induced immune thrombotic thrombocytopenia (VITT), and the symptoms it usually presents are severe headache, abdominal and leg pain, and shortness of breath.

The Technology Networks team sat down with Dr Saskia Middledorp, Professor of Internal Medicine at Radboud University, to discuss what is currently known about the VITT and in particular how this condition emerges in those rare cases.


“It’s pretty clear that at least a large proportion – and we don’t know how large that proportion is – of patients who have this vaccine-induced thrombosis combined with low levels of platelets [..] form antibodies against platelet factor 4. Platelet Factor 4 actually is released by activated platelets, and then an immune reaction forms antibodies that complex with the platelet factor 4. And by doing so, they can bind to the platelets and activate them even further, leading to thrombo-generation,” Dr Middledorp explained in the video interview. “I call that a procoagulant storm.”  

A blood clot is scientifically known as a thrombus: a coagulated mass of fibrin, platelets, and red blood cells that can plug a blood vessel. Dr Middledorp explains that we don’t yet know why the adenovirus-based vaccines seem to instigate an immune response against platelet factor 4, a protein released by platelets, but that such a reaction has been seen in other interventions.

Dr Middledorp also further quantifies the increase of blood clots in people taking the contraceptive pill, in COVID patients, and how an understanding of VITT is evolving rapidly. You can check out the full interview below:


At the time of writing, adenovirus-based vaccines remained suspended in a small number of countries.

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