Does the Astrazeneca Covid vaccine cause blood clots?

Does the Astrazeneca Covid vaccine cause blood clots?

March 11, 2021

THE ASTRAZENECA Covid vaccine has been proven safe and effective.

But today EU health officials said they are investigating a spate of cases of blood clotting in people who have recently had the jab, including one death.

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Seven European countries have halted the use of the vaccine made by AstraZeneca, developed by the University of Oxford.

Here is what we know so far.

Does the Astrazeneca Covid vaccine cause blood clots?

At the moment, it can’t be said for certain whether this is the case or not.

The EMA’s Pharmacovigilance Risk Assessment Committee (PRAC) is conducting an investigation.

It will be looking at whether the blood clotting events were caused by the coronavirus jab, or if it is just a coincidence.

But it said for now, “there is currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine”. 

The EMA also said a “quality defect is considered unlikely at this stage”.

Dr Jon Gibbins, a professor of cell biology at the Institute for Cardiovascular and Metabolic Research, University of Reading, told The Sun: “I think that currently it is too early to be definitive on this question.

"It is understandable that people are concerned, but so far the numbers of clots associated with this report are low.”

Where has this come from?

The cases of blood clots in those vaccinated appears to link back to the same batch of doses.

The EMA said the batch – labelled ABV5300 and comprised one million doses – had been delivered to 17 EU countries.

These were; Austria, Bulgaria, Cyprus, Denmark, Estonia, France, Greece, Iceland, Ireland, Latvia, Lithuania, Luxemburg, Malta, the Netherlands, Poland, Spain, Sweden.

One person has died of multiple thrombosis – formation of blood clots within blood vessels – 10 days after their vaccine.

The Danish Health Authority said the death was in Denmark. 

The EMA reported a second patient was diagnosed with pulmonary embolism – where blockages form in the arteries in the lungs – but is now recovering.

It said as of Tuesday this week, two other clotting conditions had been identified in patients that had received a dose from the same batch – but it was not clear what country they were in from the report.

Dr Gibbins said: “It is not impossible that a single batch of vaccine could have issues, although stringent quality controls are in place, and batches are checked very carefully before they are released for use.”

How many people have been affected?

The EMA said “the number of thromboembolic events in vaccinated people is no higher than that seen in the general population”.

As of 9 March 2021, 22 cases of thromboembolic events had been reported among the three million people vaccinated with the vaccine in the European Economic Area.

Dr Gibbins said this number was “low”. 

“Venous thrombosis (one type of thrombosis) affects about 1-2 people per 1,000 per year.

“The EMA identified 22 cases of thrombosis from around three million individuals that have received the vaccine – and this is much lower than the 1 – 2 per 1,000.

“Therefore, if there is a link between the vaccine and clotting, this risk is likely to be very small indeed.”

Why have some countries stopped using the AstraZeneca vaccine?

Two countries – Denmark and Iceland – have temporarily stopped using the vaccine entirely.

Austria, Estonia, Lithuania, Luxembourg and Latvia have suspended use of doses from the one batch at the source of concern.

Prof Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene & Tropical Medicine, said this was a “super-cautious approach”.

He added that it may be too cautious, in fact.

“[If] this action stops some people getting the vaccine who are then vulnerable to Covid-19, then it is a mistaken use of precaution.”

Could it just be a coincidence?


There are several risk factors for blood clots, including sitting for long periods of time, smoking and drinking alcohol.

They affect people of all ages, and can go unnoticed if the patient doesn’t know the signs. 

Prof Evans suggested it could even be that the patients who had blood clotting actually caught Covid-19.

“We know that Covid-19 disease is very strongly associated with blood clotting and there have been hundreds if not many thousands of deaths caused by blood clotting as a result of Covid-19 disease.  

“The first thing to do is to be absolutely certain that the clots did not have some other cause, including Covid-19.”

It is possible to catch the coronavirus in the first two to three weeks after receiving the first dose, as the body has not developed immunity yet.

Even some people who have received the second dose – and therefore have the maximum level of protection – will catch Covid.

Causes of blood clots

There are a variety of things that can cause deep vein thrombosis (DVT) – when you get a blood clot (a sticky mass of blood cells) in a vein that is deep below the skin.

These include:

  • Inactivity- when you are inactive your blood tends to collect in your lower body, your calfs for example. If you are inactive for a substantial length of time your blood can slow down significantly increasing your risk of DVT
  • Hospital- Long surgical procedures to the leg, hips or abdomen, and long recovery time where you are largely inactive
  • Blood vessel damage- Injuries such as broken bones or severe muscle damage can damage blood vessels, narrowing them and making a clot more likely. Vasculitis (inflammation of the blood vessels) can also put you at risk
  • Pregnancy – During pregnancy blood clots more easily to prevent too much blood loss while giving birth. Clots can also appear up to six weeks after giving birth.
  • Contraceptive pill (combined) and Hormone Replacement Therapy- The contraceptive pill and HRT both contain the female hormone oestrogen which can cause blood to clot more easily than normal.
  • Others- You are at a higher risk if you: smoke, are overweight, don’t drink enough or are aged 60 plus (particular if you have a condition that restricts your mobility)

Should we be worried?

Dr Gibbins said: “At this point we need to be careful not to cause unwarranted panic or resistance to vaccination.

“Of course it is important that this is looked into thoroughly, but at this point I am not overly concerned.”

Health regulators in the UK – the Medicines and Healthcare products Regulatory Agency (MHRA) – also struck a reassuring tone.

It said: “This is a precautionary measure by the Danish authorities. 

“It has not been confirmed that the report of a blood clot was caused by the AstraZeneca COVID-19 Vaccine. People should still go and get their COVID-19 vaccine when asked to do so.”

Has anyone given the jab in the UK had a blood clot?

There have been no reports of blood clotting after receiving the AstraZeneca/Oxford vaccine in the UK.

The batch under investigation did not supply the UK. 

The MHRA said it would be keeping an eye on the situation and that Brits should still take their jab.

Dr Phil Bryan, MHRA Vaccines Safety Lead said: “Vaccine safety is of paramount importance and we continually monitor the safety of vaccines to ensure that the benefits outweigh any potential risks.

“Blood clots can occur naturally and are not uncommon. More than 11 million doses of the Covid-19 Vaccine AstraZeneca vaccine have now been administered across the UK.

“Reports of blood clots received so far are not greater than the number that would have occurred naturally in the vaccinated population.

“The safety of the public will always come first. We are keeping this issue under close review but available evidence does not confirm that the vaccine is the cause.

"People should still go and get their Covid-19 vaccine when asked to do.”

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