AstraZeneca COVID-19 vaccine and blood clots – let’s go for facts first

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Next up on the COVID-19 vaccine hit list is that the AstraZeneca (Oxford University) vaccine causing blood clots (thrombus). Spoiler alert – no it probably doesn’t.

We should just assume that every single “adverse event” whether or not they’re actually linked to any COVID-19 vaccine will involve the post hoc ergo propter hoc fallacy, which states that because one event precedes another event, they must be linked. In this case, are blood clots related to the AstraZeneca COVID-19 vaccine?

Spoiler alert – the answer is no. That would make this a short article, but real science requires real explanation, so here we go.

The AstraZeneca COVID-19 vaccine and clots – observations

According to AstraZeneca, 15 reports 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 received the vaccine in the UK and Europe.

To put this into some perspective, let’s do the math:

  • For deep vein thrombosis, that means the observed risk after the AstraZeneca COVID-19 vaccines is 0.88 per 1 million vaccinated individuals.
  • For pulmonary embolism, the observed risk would be around 1.29 per 1 million vaccinated individuals.
  • The overall risk of these two types of blood clot events is about 1 in 1000 in the general population, substantially higher than the “risk” after the AstraZeneca COVID-19 vaccine. I’m not saying that getting the vaccine prevents blood clots, but a bad use of statistics would lead one to such a conclusion.

So far, the large, well-controlled clinical trials for the AstraZeneca and JNJ (which is also based on adenovirus vector technology) have not shown any increased risk for blood clots.

Yes, 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 Pfizer, Moderna, JNJ, and AstraZeneca vaccines. All four of these vaccines induce the production of one small part of the virus, 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.

Finally, we don’t have any information on these 37 patients – are there confounders like high risk for blood clots, such as smoking, obesity, diabetes, and other comorbidities for thrombus? Those may be more important to the risk of clots than the AstraZeneca vaccine, and that’s why we investigate each claim to determine if there is a potential link.


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 there is a link between the vaccine and clots, the incidence is so rare that the benefits, preventing COVID-19, far exceed this supposed risk of the vaccine. One of the anti-vaccine tropes is to convince us that a vaccine has to be perfect, but that’s not how the world of science works.

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 crap.

As a result of these blood clot observations, several countries in Europe have suspended (not canceled, not ended, not thrown into the trash) the use of the AstraZeneca COVID-19 vaccine. These countries include Germany, France, Spain, Italy, Ireland, and the Netherlands.

Irish Prime Minister Micheal Martin stated:

There is no causal effect established or anything like that yet, but as a precautionary move in line with the precautionary principle and in an abundance of caution, our clinical advice was to pause the program whilst the EMA does a review of this. This is an unwelcome pause but nevertheless, I think it’s important that we take heed of the advice we have received.

Apparently, the decision to suspend vaccinations is in the abundance of caution, but I don’t think it’s supported by the facts. These clotting issues are so rare that I cannot believe that the public health authorities didn’t weigh the risk of clots to the risk to the population of disease.

I think there are other issues with the AstraZeneca COVID-19 vaccine that have delayed it from submitting an Emergency Use Authorization to the US FDA, specifically the design of their clinical trial. But that has nothing to do with blood clots.

At this point in time, there just doesn’t seem to be overwhelming clinical or scientific evidence that there is a causal link between the vaccine and blood clots. But as more data is gathered, maybe it becomes more of a concern. But right now, the benefits of the vaccine far outweigh any risks, real or imagined.


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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!