Skip to content
Home » Is the AstraZeneca COVID-19 vaccine linked to blood clots? EMA provides guidance

Is the AstraZeneca COVID-19 vaccine linked to blood clots? EMA provides guidance

Last updated on August 15th, 2021 at 03:35 pm

I have previously written about whether thrombosis (formation of blood clots) is linked to the AstraZeneca COVID-19 vaccine – my conclusions were that they probably weren’t. However, governments and the European Medicines Agency (EMA) are starting to get me very concerned about an issue with this vaccine.

Recently, the government of Quebec recently decided that the AstraZeneca vaccine will only be used on individuals 55 years and older. Of course, this caused some parts of the anti-vaccine world to froth at the mouth claiming the vaccine isn’t safe.

Then, on 6 April 2021, the European Medicines Agency (EMA) is reporting a plausible link between the AstraZeneca vaccine and thrombotic events. I will discuss this in detail below.

I think that many of the adverse events that are claimed to be associated with any of the COVID-19 vaccines involve the post hoc ergo propter hoc fallacy, which states that because one event precedes another event, they must be linked. It is entirely possible that thrombosis occurs after vaccinations because of random chance rather than actual correlation (let alone causation). However, thrombosis that is temporally associated with the AstraZeneca vaccine may be troubling.

Because the AstraZeneca COVID-19 vaccine is one of the four main vaccines (along with those from Pfizer, Moderna, and JNJ) to be given in the USA, Canada, the EU, Australia, and New Zealand, I want to make sure that the potential of a causal link to blood clots are given a thorough analysis.

AstraZeneca COVID-19 vaccine thrombosis
Chicoutimi, Saguenay, QC, Canada – Photo by Andre Ouellet on Unsplash

What is the AstraZeneca vaccine?

The AstraZeneca COVID-19 vaccine is very different than the Pfizer and Moderna mRNA vaccines but is similar to the JNJ vaccine. The vaccine still induces an adaptive immune memory response to the S-protein of the SARS-CoV-2 virus which causes COVID-19.

The AstraZeneca COVID-19 vaccine utilizes a recombined adenovirus vector, chimpanzee ChAdOx1, which causes the production of the S-subunit of the SARS-CoV-2 virus which induces an immune response to that S-subunit. Basically, the adenovirus vector “carries” the genes for the S-subunit to the cell which will reproduce the protein, then inducing the immune response.

Adenovirus-based vaccines have been investigated for several decades. In fact, JNJ has received approval for an Ebola adenovirus vaccine in July 2020, so the technology did not just appear suddenly to be used to fight COVID-19-19. However, like the mRNA vaccines, these adenovirus vaccines, also used by JNJ, can be quickly developed to deliver the most important antigen on the SARS-CoV-2 virus, which is the S-protein.

There is an advantage to adenovirus-based vaccines – they are much less fragile than mRNA vaccines because they are based on DNA which is more rugged than RNA.

Once the AstraZeneca vaccine is injected into the arm, the adenoviruses enter cells and moves to the nucleus, where the cell’s genes (DNA) are located.

The adenovirus then injects its DNA into the nucleus. The adenovirus is engineered so it can’t make copies of itself, but the gene for the coronavirus spike protein can be read by the cell and copied into a molecule called messenger RNA, or mRNA.

At this point, the pharmacology of the adenovirus vector vaccines are similar to the mRNA vaccines.

Normally, during the process called transcription, RNA polymerase makes a copy of a gene from its DNA to a corresponding mRNA fragment whenever required by the cell. In other words, the mRNA sequences in the cell usually correspond directly to the DNA sequences in our genes. These mRNA sequences “carry” that genetic message to a ribosome for translation, where tRNA triplets, which code for one amino acid, attach to the appropriate mRNA triplet, adding one amino acid to the protein chain. 

As in DNA, genetic information in mRNA is contained in the sequence of nucleotides, which are arranged into codons consisting of three ribonucleotides each. Each codon codes for a specific amino acid, except the stop codons, which terminate protein synthesis.

Like with mRNA vaccines, the adenovirus does not change the genetic code of any of the 50 trillion cells that are in a human. All that happens is that the adenovirus injects DNA that is coded for the S-protein and the cell produces mRNA from that DNA that then causes the ribosomes to produce the S-protein.

Those S-proteins migrate to the surface of the cell which is then recognized by the immune system as foreign invaders. The immune system then remembers those antigens – when the actual SARS-CoV-2 virus attacks, the immune system is ready to attack.

The AstraZeneca COVID-19 vaccine also has one additional advantage over the mRNA vaccines – the adenovirus itself provokes the immune system to activate immune cells that are nearby. This leads to the immune system reacting more strongly to the spike proteins.

EMA reports plausible link

As I noted above, the European Medicines Agency, the FDA for the European Union, believes that there is a plausible link between the AstraZeneca vaccine causes rare but sometimes deadly blood clots in a tiny number of those vaccinated.

Like I mentioned above, the EMA believes that a rare condition called cerebral venous sinus thrombosis (CVST), a clot that stops blood from draining from the brain. Regulators have said it is occurring among those who have received the AstraZeneca COVID-19 vaccine at a rate above what they’d expect to see in the normal population.

The EMA is reporting a total of 169 cases of CVST among the 34 million people given the AstraZeneca vaccine across Europe as of 4 April 2021. Additionally, there have been 52 other cases of rare thromboses. The EMA reported that they based its scientific review on an initial 62 cases and 18 deaths up until March 22, but continued reports did not change their assessment.

To put this into some perspective, let’s do the basic math. The risk of these uncommon thromboembolic events is around 6.21 per 1 million people given the AstraZeneca COVID-19 vaccine. On the other hand, the risk of death from COVID-19 is around 1,000-2,000 per 1 million who contract the disease, which is around 200X greater risk than the risk of thrombosis from the vaccine.

In addition, the EMA is reporting that the risk is higher among women under 60, although they were unable to explain the reason for this observation.

The risk of CVST in the general unvaccinated population in the USA is around 0.5 to 1 in 1 million, so the observed risk in those vaccinated with the AstraZeneca vaccine is substantially higher.

Yes, the SARS-CoV-2 virus seems to cause thrombi in many patients, which might lead one to believe that there is some biological plausibility to the blood clot issue. We don’t appear to observe these thrombosis events with the Pfizer, Moderna, and JNJ vaccines, so it is possible that either there is an ingredient in the AstraZeneca vaccine that leads to this increased risk.

Or it may be possible that the AstraZeneca vaccine codes for the spike protein in such a manner that it mimics the clotting issues observed with a SARS-CoV-2 infection.

Obviously, a lot more research is necessary to confirm any of this.

astrazeneca COVID-19 vaccine blood clots

AstraZeneca vaccine and blood clots – Canada

As I mentioned above, the government of Québec has stopped using the AstraZeneca COVID-19 vaccine for those 55 and under, pending further investigation about whether it causes thrombus. They are following the Canadian guidelines regarding this issue, especially since the US Government has decided to provide this vaccine to Canada and Mexico.

The government of Canada stated that:

This adverse event is being referred to as Vaccine-Induced Prothrombotic Immune Thrombocytopenia (VIPIT). This entity is associated with the development of antibodies that “activate” platelets, which stimulate the formation of clots and result in thrombocytopenia. The mechanism of action is similar to heparin-induced thrombocytopenia (HIT). The exact mechanism by which the AstraZeneca vaccine triggers VIPIT is still under investigation. At this time, no other risk factors have consistently been identified in patients who develop VIPIT. This adverse event has not been identified following receipt of mRNA COVID-19 vaccines to date.

And after new data showed a slight increase in the risk of sinus vein thrombosis (SVT), a very rare thromboembolic event, health authorities in various German states made the decision to temporarily halt vaccinations for younger people after receiving more data about these clots.

The Paul Ehrlich Institute said a total of 31 SVT events were reported by March 29 out of some 2.7 million doses of the AstraZeneca COVID-19 vaccine that have been administered across the whole of Germany. Nine of the people died and all but two of the cases involved women, who were aged 20 to 63, it said.

The Institute also reported:

…experts of the Paul-Ehrlich-Institut now see a striking accumulation of a special form of very rare cerebral vein thrombosis (sinus vein thrombosis) in connection with a deficiency of blood platelets (thrombocytopenia) and bleeding in temporal proximity to vaccinations with the COVID-19 vaccine AstraZeneca.

AstraZeneca COVID-19 vaccine blood clots


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

Admittedly, I am deeply troubled by the data out of Europe that shows a strong correlation between SVT and CVST and the vaccine, especially in some women.

If this link is supported by more data in more countries (observations in one country can be affected by all kinds of bias), then AstraZeneca must be transparent in what is being observed and give warnings to healthcare workers to prepare for it.

Do I distrust AstraZeneca? I am getting really close to that. With their massive issues with their clinical trial design (which would not have any impact on safety just effectiveness), the issues with clotting, and their lack of an application to the Food and Drug Administration for an Emergency Use Authorization, I’m moving from a lukewarm supporter of this vaccine to stating that a lot more research needs to be done and fast.

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.

But this is a serious risk, so despite the effectiveness of the vaccine, public health authorities and governments need to weigh that risk with the benefits to their country in reducing COVID-19. The risk of blood clots with the AstraZeneca COVID-19 vaccine is still far lower than the risk of dying of COVID-19. And it is much much lower than the risk of serious long-term effects from the disease.

Right now, there are three other very effective and very safe vaccines from Pfizer, Moderna, and JNJ that will do the job. Now, some parts of the world may have bungled their vaccine rollout, so the AstraZeneca vaccine may be the only choice.

Would I get the vaccine? No, unless there were no choices. The risk of these blood clots after receiving the AstraZeneca COVID-19 vaccine is just high enough that I’d rather get the other COVID-19 vaccines. But if there is no choice, I would take the AstraZeneca version, because I know the risks of COVID-19 are just too high.

Finally, the anti-vaxxers are going to claim that this 6 in a million risk means that all vaccines are bad. No, it doesn’t, it only shows that science is damn good at finding causation and correlation. Instead of scaring people about this vaccine, they should feel great comfort that science and medicine are constantly self-correcting when new data is available.


  1. My calculated numbers for these risks are based on the infamous back of the napkin analyses. Once peer-reviewed papers are published, we will know precise risk difference between vaccinated unvaccinated groups.



Michael Simpson

Don’t miss each new article!

We don’t spam! Read our privacy policy for more info.

Liked it? Take a second to support Michael Simpson on Patreon!
Become a patron at Patreon!