Does the AstraZeneca Covid vaccine cause blood clots?

THE AstraZeneca Covid vaccine has been proven safe and effective, according to the UK regulator.

But there are ongoing investigations to determine if there is a link or if the cases of blood clotting are a coincidence.

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What blood clots have been found and how many?

Thirty cases of the rare blood clotting have been seen in the UK out of 18.1 million doses given, as of April 2, of the Oxford vaccine.

But the Medicines and Healthcare Regulatory Agency (MHRA) stressed the risk was “very small” and the benefits of the vaccines against Covid continue to outweigh any risks”.

European regulators have made the same conclusions while looking into cases across the continent, while the World Health Organization has told countries to keep jabbing.

The International Society on Thrombosis and Haemostasis has also recommended that all eligible adults continue to receive their Covid-19 vaccine.

When the first cases were reported, at least 20 European countries halted use of the AstraZeneca (AZ) vaccine, developed by the University of Oxford, in a knee-jerk reaction.

Most resumed vaccine programmes, but France and Germany have said it should not be given to younger patients who are thought to be more at risk of clots.

Overall the numbers of cases are minimal compared to the millions of doses given out.

Regulators are looking closely at a type of blood clot called cerebral venous sinus thrombosis (CVST) – a serious condition which occurs when a blood clot forms in the brain’s venous.

These cases have occurred with low levels of blood platelets, called thrombocytopenia, which is said to be very unusual.

The European Medicines Agency (EMA) – the first to raise the alarm – gave an update on March 31 in the context of its ongoing review of "very rare cases of unusual blood clots associated with low numbers of platelets".

It said it was investigating 62 cases in 9.2 million people given the AZ vaccine, 44 of which are understood to be in Europe. The figure does not include all cases reported, however.

In the UK, the MHRA said on April 1 there have been 22 cases of this rare CVST kind of clot, and eight other thrombosis events.

It is known that at least one of the people who have had CVST with thrombocytopenia in the UK has died.

No cases of CVST with thrombocytopenia have been reported in those given the Pfizer/BioNTech vaccine. However at least two people have had CVST alone.

Overall, more people in the UK have had a blood clotting event of any kind after the Pfizer vaccine compared to the one from AZ.

German regulators have reported 31 cases of CVST with thrombocytopenia among the nearly 2.7 million given a jab, nine of whom have died.

Does the AstraZeneca Covid vaccine cause blood clots?

The EMA and the MHRA have said there is not an “overall increased risk” in blood clotting conditions.

AZ has also said its review of data has shown no evidence of blood clotting in those given the vaccine.

Ann Taylor, chief medical officer at the firm, said the number of cases of blood clots reported – fewer than 40 in 17 million people in Europe – is lower than the hundreds of cases that would be expected among the general population.

But as for a rare type of blood clotting of the brain, it can’t be said for certain whether these were or were not caused by the AZ vaccine.

But there is no evidence so far to suggest that is the case.

Speaking to journalists on March 18, Dr Phil Bryan, MHRA vaccine safety lead, said: “What we don't know is whether these cases have been caused by the vaccine.

"We are working closely with experts to try and gather more information to determine this, because these illnesses do very rarely happen naturally."

EMA executive director Emer Cooke was asked if a link between the rare cases of blood clots and the vaccine is likely on March 31 and said: "At the moment at this stage of our investigations the link is possible and we cannot say any more than that at this point."

While a definitive link cannot be ruled out, senior regulators have said the benefits of having the vaccine far outweigh any potential risks and have declared it "safe and effective".

Why have some countries stopped using the AstraZeneca vaccine?

The list of countries that suspended the AZ vaccine grew over mid-March, reaching at least 20 in Europe alone.

The move was said to be extremely pracautionary while more information was gathered.

Some scientists said the "super-cautious approach" taken by some leaders may come at a cost.

However, it soon became clear some countries had halted use as a political move.

Health officials in Italy admitted the ban was political, after other EU nations including Germany and France suspended use of the jab.

French Europe Minister Clement Beaune admitted the suspension heaped "political pressure" on AstraZeneca amid the ongoing supply dispute.

Most countries did a U-turn and resumed vaccination after the EMA said the jab was "safe and effective", including Italy and Sweden.

But there are still some restrictions in place – just days after France said it would continue "without delay" it said it would only give the jab to those over 55 years old.

Germany has also banned it for under 60-year-olds, while Canada has suspended use in people under 55 due to the concerns raised in Europe.

What has AstraZeneca said?

AstraZeneca has previously said its own review found no evidence of an increased risk of pulmonary embolism, deep vein thrombosis or thrombocytopenia, in any defined age group, gender, or in any particular country.

In clinical trials for the jab, the number of clotting incidents was small and “lower in the vaccinated group” than in those who were unvaccinated, it added.

The firm said international regulators had found the benefits of its jab outweigh any risks.

However, it said it will continue to analyse its database to understand “whether these very rare cases of blood clots associated with thrombocytopenia (low blood platelet count) occur any more commonly than would be expected naturally in a population of millions of people”.

Could it just be a coincidence?

Yes. Some experts have said the cases are most likely a coincidence.

Dr Peter English, immediate past chairman of the British Medical Association’s public health medicine committee, said: “If you look for excesses of cases in a wide range of rare conditions, you will find some, due to the play of chance.

“There is no evidence that these cases are caused by vaccination, rather than simply associated with it, in people who would have had such conditions anyway."

There are several risk factors for blood clots in general, 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. 

Robert Storey, professor of cardiology, University of Sheffield, said: "Because blood clotting illnesses are common, it is expected that many people, out of the millions vaccinated, will suffer one of these by chance after receiving a Covid-19 vaccine."

Experts have said it could even be that the patients who had blood clotting actually caught Covid-19 before their jab had time to kick in.

Prof Evans said: “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.”

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?

Health regulators in the UK have struck a reassuring tone and British scientists have been largely unconcerned.

The Prime Minister Boris Johnson defended the vaccine and said it was both safe and effective.

Writing for The Sun, the Health Secretary Matt Hancock said: "I want to reassure Sun readers — there is no evidence that vaccines caused these clots.

"Don’t just take my word for it — this is the view of the UK’s medicines regulator the MHRA, the European Medicines Agency, who reviewed the evidence just yesterday, the World Health Organisation and countless doctors and clinical experts who have made their views clear in these pages and elsewhere."

Professor Adam Finn, from the University of Bristol and a member of the Joint Committee on Vaccination and Immunisation, said many vaccines currently in use “do have very rare, unexpected serious side effects".

"But we still use them because the balance of risk and benefit is greatly in favour of using them,” he added.

“Just as we all get up in the morning and go to work and take a mortal risk … we find that acceptable because we might die in a car accident or be knocked down by a bus.

"We have to get used to the idea that using vaccines and drugs and medicines is not without risk, but they’re very, very small risks, and the risks of not using them is obviously much greater.

“I think one thing we can say at this moment is that the benefits (of the vaccine) outweigh the risk."

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