AstraZeneca Vaccine Blood Clot Risk : How Dangerous Is It?

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What are the risks of getting a dangerous blood clot from the AstraZeneca COVID-19 vaccine, and just how safe is it?

Let’s take a look at what the latest findings show!

 

AstraZeneca COVID-19 Vaccine : A Quick Primer

The AstraZeneca Vaxzevria vaccine, codenamed AZD1222, is a viral vector vaccine against COVID-19.

It uses a chimpanzee adenovirus – ChAdOx1 – which has been modified to prevent replication, to introduce a DNA sequence of the SARS-CoV-2 spike protein.

Once injected, the vaccine enters the cell and “teaches” it to produce the SARS-CoV-2 spike proteins and express them on its surface.

These spike proteins triggers the immune response to create antibodies that will protect you against the real SARS-CoV-2 virus.

 

AstraZeneca Vaccine Safety Concern : Blood Clot Risk

Scientists are now quite certain that the AstraZeneca vaccine can induce a rare side effect with a variety of names :

  • Thrombosis with Thrombocytopenia Syndrome (TTS)
  • Vaccine-induced Thrombotic Thrombocytopenia (VITT) or even
  • Vaccine-induced Prothrombotic Immune Thrombocytopenia (VIPIT).

In a very small number of cases, the AstraZeneca vaccine can trigger the immune system to create anti-PF4 (Anti-Platelet Factor 4) antibodies.

These anti-PF4 antibodies attack the platelets, causing them to clump together and blood clots to form.

What Are The Symptoms?

Patients who develop these blood clots will complain of these symptoms 4 to 30 days after receiving the AstraZeneca vaccine :

  • Severe headache that
    – does not improve with simple painkillers
    – gets worse when lying down
  • Neurological changes
    – blurred vision
    – speech difficulties
    – drowsiness
    – seizures
  • Nausea and vomiting
  • Persistent abdominal pain
  • Back pain
  • Shortness of breath
  • Leg pain or swelling
  • Petechiae (tiny blood spots under the skin)
  • Easy bruising or bleeding

What Causes These Antibodies To Form?

There were concerns that this may be a cross-reaction to the SARS-CoV-2 spike protein. After all, SARS-CoV-2 infections also cause blood clots.

However, this pre-print study concluded that the antibodies against SARS-CoV-2 spike protein do NOT cross-react with platelet factor 4.

Therefore, it seems likely that the adenovirus vectors used in both AstraZeneca and Johnson & Johnson vaccines could be the trigger for this reaction.

After all, other vaccines that target the same SARS-CoV-2 spike protein – like the Pfizer and Moderna vaccines – do not have this risk.

Who Is Most At Risk?

Based on the initial case reports of just 5 cases, it appeared to happen mostly in women below 60 years of age, with a time-to-onset of 2 weeks or less following vaccination.

However, later analysis could not stratify the risks according to gender, only by age.

So exactly how common is this blood clot risk?

 

AstraZeneca Vaccine Blood Clot Risk : TLDR Summary

While it is important to recognise that the AstraZeneca vaccine increases the risk of thrombotic thrombocytopenia, it is also important to recognise that the risk is very small.

In fact, the risk of developing blood clots is much higher in COVID-19 patients – up to 41% in positive patients, and up to 95% in severe COVID-19 cases.

Blood clots can also develop (through different mechanisms) with smoking and the use of oral contraceptive pills (OCP).

We created this table to better demonstrate the risk to you.

Risk Difference
Thrombocytopenia in
severe COVID-19 cases		 Up to 95% 45,238X
Thrombocytopenia
in COVID-19 cases		 Up to 41% 19,524X
Dying from COVID-19 Infection 0.25% to
10%		 119X to 4762X
Thrombosis from OCP
(Adult) 		Up to 0.46% 219X
Thrombosis from Smoking 0.18% 85.7X
Thrombosis from OCP
(Adolescents) 		Up to 0.01% 4.8X
Thrombotic Thrombocytopenia
(AstraZeneca Vaccine)		 0.0021% Baseline

As you can tell, the risk of getting thrombotic thrombocytopenia is exceedingly small.

Should you get the AstraZeneca vaccine?

The rule of thumb for now seems to be thus :

  • in countries with high incidence rates (over 1%) – you should definitely get it.
  • in countries with moderate incidence rates (~0.5%) – people 30 years or older should get it, unless there is another alternative vaccine.
  • in countries with low incidence rate (< 0.01%) – people 60 years or older should get it.

More importantly, that we are aware of this particular side effect, both doctors and vaccine recipients can look out for the symptoms and quickly seek treatment.

 

Some Countries Limit AstraZeneca Vaccine To 60 Years + Older

You may have noticed that many countries like Germany, Ireland and Italy are recommending that the AstraZeneca vaccine be used for people who are 60 years or older.

This is because the initial CHMP assessment report (see below) showed that the benefits greatly outweigh the risks for the age groups of 60 years and above in the worst case scenario.

Hence, out of an abundance of caution and because they have alternative vaccines, they are limiting the use of the AstraZeneca vaccine to people who are 60 years and older.

This assessment was based on a very low incidence rate – just 55 people infected out of 100,000 people, so it is not reflective of the real world situation in many countries.

 

AstraZeneca Vaccine Blood Clot Risk : By Age

On 23 April, the EMA Committee for Medicinal Products for Human Use (CHMP) released their Assessment Report on the blood clot risk of the AstraZeneca Vaxzevria COVID-19 vaccine.

Due to the low number of reported cases, they could not identify risk factors for TTS (Thrombotic Thrombocytopenia) associated with the vaccine, and “it was not possible to further stratify risk by sex“.

Based on just 142 reported cases, they were able to calculate the interim risk by age.

The age groups at greatest risks seem clear-cut :

  • 40-49 years old : 0.0021%
  • 20-29 years old : 0.0019%
  • 30-39 years old : 0.0018%

We should point out that that’s the risk of developing blood clots with low platelet counts. It is NOT the risk of dying from the blood clots.

 

AstraZeneca Vaccine Blood Clot Risk : Worst Case Scenario

CHMP analysed the risk of blood clots versus the benefits of the AstraZeneca COVID-19 vaccine in high, medium and low exposure situations.

Let’s look at the WORST CASE SCENARIO, by looking only at the low exposure situation.

Hospitalisations

This visual representation of the risk of getting blood clots versus the number of COVID-19 hospitalisations prevented by the vaccine makes it easier to do the risk-benefit analysis.

It is pretty obvious, that the benefits easily outweigh the risks for 50 years and above, when it comes to preventing hospitalisations.

ICU Admissions

When it comes to preventing ICU admissions (severe COVID-19 cases), the data shows that the benefits far outweigh the risks for 60 years and above.

Deaths

And this is arguably, the most important criteria – preventing deaths. Similarly like with ICU admissions, the benefits far outweigh the risks for 60 years and above.

 

AstraZeneca Vaccine Blood Clot Risk : High Risk Scenario

While the low COVID-19 exposure risk assessment is great for a “worst case scenario”, it is not representative of the real world situation.

In particular, Malaysia right now has a COVID-19 incidence rate (attack rate) of about 1.3%. So let’s take a look at the CHMP’s high exposure scenario.

Hospitalisations

This visual representation of the risk of getting blood clots versus the number of COVID-19 hospitalisations prevented by the vaccine makes it easier to do the risk-benefit analysis.

It is pretty obvious, that the benefits easily outweigh the risks at all age groups, when it comes to preventing hospitalisations.

ICU Admissions

When it comes to preventing ICU admissions (severe COVID-19 cases), the data shows that the benefits far outweigh the risks for all age groups.

Deaths

And this is arguably, the most important criteria – preventing deaths. In this case, the benefits greatly outweigh the risks for 40 years and above.

 

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