• Problems with a rare condition affecting the smallest of cells in the blood called platelets has been occurring at increased rates after vaccination with the Astra-Zeneca vaccine.
  • This leads to clotting and bleeding problems, including clots in the veins in the brain
  • Diagnosis is made with simple blood tests and treatment is highly effective.
  • The risk varies ranges from 1 in 150,000 in Germany to 1: 600,000 in the UK.
  • This means about 99.99958% of vaccinated people are not affected by this problem.
  • Some nations are restricting the use of the AZ vaccine to the less at risk elderly population.

So what’s happened?

The clotting problems experienced in Germany are defining the news for Astra Zeneca right now. The Germans have reported a condition called Central Venous Sinus Thrombosis in 31 people after 2.7 million doses of the Astra Zeneca vaccine as opposed to its normal incidence of about 13 a year. This excess risk means the chance of it happening to to a vaccinated individual is 1:150,000. In the EU as a whole, the risk, as far as we know right now, is 1:210,000, less in the UK who have vaccinated far more elderly people. While initially recommending the vaccine only for the under 60’s, the Germans are now recommending it only for the over 60’s as CVST primarily affects younger people.

What is CVST?

Central Vein Sinus Thrombosis is a condition I only saw once in my time as a Doctor. It is an unusual sort of stroke, caused by clotting in a vein draining blood away from the brain. (The more usual sort of stroke is due to clots in or bleeding from an artery supplying blood to the brain). It can lead to odd symptoms of headaches, and problems affecting arms or legs, speech or sight. It can also cause convulsions. Given its rarity, diagnosis can be tricky and delays, like all conditions, causes problems. Delay should be minimised by awareness of the problem and outcome improved as there are effective treatments.

A study of the first 9 patients in Germany, sheds some light on what is going on. The average age was 36, and 8 patients were women. They had all developed antibodies to something called Platelet Activation 4. This leads to activation of platelets which causes clotting in the veins. 7 of the 9 occurring in the veins draining the brain, one in the spleen and one in the lungs. Sadly, four died. The authors suggest the term “Prothrombotic Immune Thrombocytopenia (VIPIT) for the condition.

Platelets are tiny cells in the blood which cause clotting and so prevent bleeding. (The cell in the middle is a white blood cell which fight infection, the others are red blood cells which transport oxygen)

It is similar to a problem which occurs rarely after use of the anti-clotting drug Heparin, called Heparin Induced Thrombocytopenia (HIT) and for which I remember screening for in the elderly two weeks after the use of the drug to reduce the risk of clots after knee surgery.

There are simple blood tests to diagnose the problem and effective treatments which, if started early enough, should lead to a good outcome. A recent review found that death occurred in 5% of those affected, and 80% recovered fully. Awareness of the risk, particularly for younger women and possibly those on hormonal contraception should mean simple screening and early treatment will largely prevent any further tragedies and reduce suffering.

How much of problem is it here?

Here in the UK the MHRA has reported 22 cases of CVST and 8 other episodes of clotting in the 18 million people vaccinated, giving a risk, as far as we know of about 1 in 600,000. Why so much lower than Germany? This might be because they restricted the use of the AZO vaccine to younger groups where the risk might be higher. There have been two cases of CVST after the Pfizer vaccine, though blood tests in these were normal. This does suggest that the Astra Zeneca vaccine is causing this problem, though it remains hard to be certain. For the elderly, the target of the vaccination campaign so far, this is less of an issue as the emerging risks of the vaccine are dwarfed by its benefits.

It is good that this has been picked up and is in the news, as there are important lessons to be learned.


Early diagnosis – The symptoms seem to begin 4- 10 days after the vaccination. Bleeding can take place and can be noticed as excess bruising or spots on the skin. Other symptoms of clotting depend on where the clots have formed, but suffice it to say, any odd symptoms the fortnight after vaccination should prompt a call to the doctor.

If you see these sort of spots on the skin, or have odd bruising, or headaches, weakness, numbness or problems with speech in the few weeks following a vaccination, then call the doctor and get tested for clotting problems

Early treatment – one problems with CVST, like many rare conditions with odd symptoms is frequently its late diagnosis. However, with reporting of the problem it means the hint of a recent vaccination should lead to an early diagnosis made by a simple blood test and prevent severe problems developing. I hope that early treatment means that there will be less deaths as time goes by.

Vaccines in the real world.

This will be the landscape beyond the clinical trials, which genuinely showed that vaccinations are safe for the vast majority of people – we now explore the tricky world of the emergence of rare problems as millions are vaccinated and how confidence in the vaccines are affected.

Of course I feel a sense of sadness that this has come up and for those affected by the problem, but also an inevitability that there continues to be no silver bullet. A perfectly effective and safe treatment for anything, vaccination included, doesn’t exist. But we have to look at the numbers. The risk is very low and it seems, lower for the elderly. If it transpires that the risk is greater for younger women and possibly those on the contraceptive pill, or in younger people with certain risk factors, then this can be included on the vaccination information for patients and inform the decision about vaccination.

Like all medical interventions, vaccinations will have a number of side effects, most wanted, some not. It’s a matter of weighing up one against the other. People affected by illness are prepared to put up with serious side effects to have a shot at a cure, but when it comes to preventing illness we have not yet experienced, then the decision is a bit more nuanced. Prevention is, however, always better than cure, if cure exists.

With the release of lockdown gathering pace in the UK it remains to be seen how this will affect the vaccination campaign over here. The risk of death might not seem such an issue for younger people, though COVID19 remains an unpleasant illness for some and the emerging long term effect of COVID19 are something to be avoided at all ages. It needs to be balanced against the risk of dying from COVID19 which for people aged 40-50 is about 1 in 1000.

People will make their own minds up, and the issue will be more clearly defined as time goes by, data comes in. Building up community immunity is also something we cannot ignore – in other words, the benefits of your vaccination for others.

Given that this problem has not affected me, or (given an average risk so far of about 1:200,000), 99.99958% of others who have had the first jab, I will certainly be having my second when my turn comes along.

2 thoughts on “Aztra Zenecas clotting problems

  1. Interesting. With any medical intervention there is always a risk. Oral polio vaccine (and I am using unverified Wikipedia numbers here) causes vaccine-associated paralytic poliomyelitis in about three per million doses. This is vastly lower than the historical rate of paralysis from ‘wild’ polio in an unvaccinated population. No-one would argue that OPV was/is a bad thing. The current arguments against mask usage and vaccination passports from a privacy and personal freedom viewpoint are absurd.

    1. Thanks for the feedback, Andrew and the point about OPV is well made. I think the summer will be interesting as we adapt and hopefully recover. There will be lots of arguments about government over-reach versus freedom. I agree many of the arguments against passports are odd. Some think they will be discriminatory – I guess thats the whole idea. There will be issues with the young at large though who will have a different risk/benefit profile to old codgers like me.

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