AstraZeneca acknowledges Covid vaccine causes clots

1 in 10,000 who took this vaccine, which also saved many lives, could suffer this side effect...
03 May 2024

Interview with 

Clare Bryant, University of Cambridge


Blood smear, blood cells


First this week, the pharmaceutical behemoth AstraZeneca has acknowledged that its widely used Covid-19 vaccine, which is branded as Covishield, can cause rare side effects - including blood clots. Covishield was developed by AstraZeneca in collaboration with the University of Oxford, and has been widely administered across the world. But what is known about the vaccine and its side effects? I went to meet Clare Bryant, an immunologist at the University of Cambridge…

Clare - The AstraZeneca vaccine took a chimpanzee adenovirus, that's the kind of virus that causes colds, but because it comes from a chimpanzee it doesn't actually cause disease in people. What they did was they introduced the spike protein gene, which is the key part of the coronavirus that everybody had the vaccines made against. They introduced that into the chimp adenovirus, and then they used that to vaccinate people. The interesting thing about that viral vector is that it can enter human cells, but it can't proliferate. So disabled virus basically enters human cells, introduces the spike protein gene, then the cells make the spike protein, and this is then passed out of the cell to form a vaccine response. Note at this point please everybody, that that does not mean that the spike protein gene integrates into the human DNA. It just sits there and gets transcribed into the protein. It's a technology that's actually been well worked. The Oxford group who worked with AstraZeneca to make the vaccine have been using this technology for many years, and indeed they used it to make a successful Ebola vaccine.

Chris - Very successful though, wasn't it? The statistic I read was that literally billions of doses were given and millions of lives were saved. So, at the end of the day, it did do a very good job, regardless of side effects which we'll come to in a minute.

Clare - Yeah, for sure. The estimate is it saved 6 million lives. They think it was about 70 to 80% effective, and that's what we needed at the time.

Chris - And what was the outcome that made people concerned about the AstraZeneca vaccine as its rollout occurred?

Clare - What occurred was that very few people had a very serious side effect. It's awful, really awful, for very few people. One in 10,000, I think, they saw this in. They ended up having some kind of clot. So what would happen is that people would produce some clots, blood clots. These could lodge in different parts of the body and then cause very serious outcomes. If it goes into your brain, of course it gives you a stroke. This is life threatening. Certainly life altering and potentially life threatening. This was the problem with the AstraZeneca vaccine. But again, very, very rare. But very, very serious when it occurs.

Chris - Do we know the mechanism of that occurring? Do we know why that one in 10,000 people had this happen to them and the other 9,999 people didn't?

Clare - Not completely, but there is some evidence beginning to emerge to try and understand what's happening. So what happens is, when you get vaccinated, you produce antibodies and you produce predominantly antibodies against the spike protein. But sometimes you can get antibodies produced against other proteins in the body. In this case, what happens in this vaccine induced thrombotic syndrome is that the antibody binds to a platelet protein. Platelets are critical for driving clotting. This then forms a big blob, effectively, and activates the platelets and the platelets then clot. Now exactly why that happens in some people and not others is not really understood. We don't really know clearly yet what parts of the vaccine may be driving this, but it is a well recognised syndrome with some vaccines, very well recognised, and that's part of the mechanism that's happening in this case.

Chris - Do you think it was unique to the Covid use of that vaccine backbone? Because you mentioned they've used it for other things like Ebola and we didn't see this. Is it just the scale of use or is it there's something special about the Covid vaccine and therefore they can carry on using the technology quite safely for other things? Because it is very good, it's very efficient, very agile and very cheap as well, which was one of the attractions. Very good for resource poor settings.

Clare - I suspect it's the scale, to be honest. It's really hard to tell. We don't know the answer to that question, I think. And the scale of rollout of the Covid vaccine was so huge that when you've got a side effect that's occurring in one in 10,000 people, then you will pick it up if you're giving millions, billions of doses. I think we don't know about things like the Ebola vaccine. I think it's possible, but we will have to wait and see unfortunately, unless we actually work out precisely what it is about the AstraZeneca vaccine. Could then the Covid vaccine be modified? Because if we do understand that mechanism and can take that out of the adenovirus backbone, then we would be looking at something that, actually, because the other side effects that occurred were the same as you would get with a cold or flu or something like that, which you also got with the Moderna vaccines. So there's work to be done, I think, on that, and I'm sure that's actively being done because potentially this technology is so fantastic for generating cheap vaccines against serious diseases.


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