COVID-19 vaccines and blood clots – a review of current science

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There have been extremely rare reports about blood clots being related to the JNJ and AstraZeneca COVID vaccines. Although science does not why this happens, researchers continue to explore why these COVID-19 vaccines might be linked to blood clots (thrombus).

A new review of the current research clears up some of the mystery, but, unfortunately, opens up some new questions. This article will examine the science, at least as far as we know, behind these blood clots and the two COVID-19 vaccines.

COVID-19 vaccines and blood clots

A few months ago, AstraZeneca reported 15 cases of deep vein thrombosis and 22 reports of pulmonary embolism, both caused by blood clots, in patients who have received their COVID-19 vaccine. These observations are out of 17 million patients who had received, at that time, the vaccine in the UK and Europe.

After reviewing these cases, researchers now call it vaccine-induced immune thrombotic thrombocytopenia (VITT), a life-threatening and mysterious condition that affects a very small number of people who have received the AstraZeneca or JNJ COVID-19 vaccines. It is estimated that VITT occurs in about 1 in 50,000 people aged under 50 who received the AstraZeneca version of the vaccine.

For a little background, these two COVID-19 vaccines use disabled adenoviruses as a ‘vector’ to shuttle mRNA, encoding for the coronavirus spike protein, into human cells. Once there, the mRNA causes the protein to be made. The immune system detects the spike protein and generates antibodies against it that are crucial for protection against coronavirus infection.

The SARS-CoV-2 virus seems to cause thrombi in many patients, which might lead one to think there is some biological plausibility to the blood clot issue. However, no live virus is used in the JNJ and AstraZeneca vaccines. These two vaccines induce the production of the spike protein for a limited time. A real infection by SARS-CoV-2 will have billions of viruses all replicating and causing havoc to the body.

Although many researchers are examining why these two COVID-19 vaccines cause VITT, no one is sure what might cause it. Furthermore, because it is so rare, it becomes difficult for researchers to determine if the vaccines are actually causing it and what might be the mechanism of action. It’s terribly frustrating.

The vaccines seem to induce an immune response against PF4, a cytokine involved in the clotting process when blood samples from vaccinated patients were reviewed. They also found that about ⅔ of the samples they received was blood clotting disorders not related to the JNJ and AstraZeneca vaccines.

The researchers have proposed several hypotheses as to why the vaccine does something to the PF4:

Of course, the evidence support any of these hypotheses is weak at best. Again, these blood clots are so rare that it makes it difficult for researchers to really narrow down the cause. It’s also possible that there’s some rare factor common to all of the people who express VITT that allows the vaccine to somehow interact with the PF4.


Of course, regulatory authorities, public health organizations, AstraZeneca, and others will not ignore this, and they will all work together to investigate whether there is an actual link between blood clots and the AstraZeneca COVID-19 vaccine.

If the link between these COVID-19 vaccines and blood clots holds up, the incidence is so low that the benefits, preventing COVID-19, far exceed this risk of VITT.

All vaccines have some risks, almost always very minor ones – anti-vaxxers frequently fail to understand the vaccine risk-benefit equation by rounding up rare events to 100% and rounding down effectiveness to 0%. This is a perfect example of the Nirvana fallacy, which states that if something isn’t perfect, it’s bad.

All countries that have approved these vaccines are monitoring the situation closely, and they have issued recommendations in case VITT is observed. For example, the CDC has issued clinical recommendations for the JNJ vaccine after a pause in its use earlier in 2021 because of blood clots.


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The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!