How big is the risk of fatal blood clots from Covishield?


(This story originally appeared in on Apr 21, 2021)

Covid patients are sharing hospital beds and in some places the dead have shared pyres. It couldn’t get worse, you think, until yet another report raises doubts about the safety of the Oxford-Astra-Zeneca vaccine.

Since February, scientists in Europe have suspected a link between the vaccine and rare but recurring cases of fatal blood clots. Some countries have stopped using the vaccine, which is called Covishield in India and Vaxzevria abroad.

The West has the luxury of choice, but for us Covishield is the mainstay of the entire vaccination drive. If you are in line for a Covid shot, should you be afraid? Should you wait for another vaccine to launch? Here are answers to some of the questions on your mind.

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Why is Europe worried?


Blood clots are a common problem. You can get one by sitting too long in one place. While these ordinary clots are easy to treat, those linked to the

vaccine are not. In fact, they combine clotting with the risk of internal bleeding.

Scientists have named this condition “vaccine-induced immune thrombotic thrombocytopenia (VITT).” Somehow, a tiny percentage of people who take the AstraZeneca vaccine seem to face this dual problem of clots and internal bleeding.

The clots can occur anywhere in the body, but the deadliest ones occur in the brain. They are called cerebral venous thrombosis (CVT) or cerebral venous sinus thrombosis (CVST).

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Covid causes more clots


The concern about the vaccine is based on a few hundred CVT cases recorded in Europe. In the European Economic Area (EU+Iceland, Liechtenstein and Norway), 169 CVT cases were recorded after 34 million vaccinations. That’s roughly 5 out of every million AstraZeneca recipients.

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Among 20 million AstraZeneca recipients in the UK – roughly the population of Mumbai – 19 died of VITT, the overall clotting/bleeding problem that includes CVT.

You might say, each one of those deaths is one too many. True, but consider the alternative. You avoid the vaccine and get Covid.

What is the risk of death now?


Besides, Covid also causes VITT and CVT, and new data shows it causes them far more often than vaccines.

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On April 15, Oxford scientists published research claiming that the risk of CVT from Covid is 8 times greater than the risk from their a vaccine.

Are we sure the vaccine is the cause?


We still don’t know for sure if the AstraZeneca vaccine ‘causes’ VITT/CVT, but international agencies are leaning towards that view.

The European Medicines Agency (EMA) has said VITT should be listed as a “very rare side effect” of the vaccine. UK authorities say the evidence of a link is strong but “more work is still needed.” But WHO has held off so far. Based on data from 190 million AstraZeneca vaccinations, it feels a causal relationship between the vaccine and VITT is “plausible” but not “confirmed”.

What should you do?


With cases shooting up day after day, the most important thing is to get your vaccine as soon as you can. If you have already had your first shot of Covishield without serious side effects, UK authorities advise you take the second on schedule.

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If at all you get VITT/CVT, the symptoms will appear 4 days to 4 weeks after your first shot. You should see a doctor “urgently”.

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Are women at greater risk?


Initial reports said women who got the AstraZeneca vaccine were at greater risk of CVT, but that does not seem to be the case now.

“The differential could be explained by the higher number of young women being given the vaccine, in large part because there are more women working in health and care professions than men,” Anna Gross writes in Financial Times. In England, 7.5% more women have got their first dose. In the under-50 age group, women are 41% ahead.

Although the contraceptive pill also causes clotting – about 1 in 1,000 women a year – professor Adam Taylor from Lancaster University writes in The Conversation: “the mechanisms behind the type of clotting linked to the vaccine and that linked to the pill appear quite different.”

So, for now, theories linking the contraceptive pill, pregnancy and hormonal replacement therapy to clots occurring after vaccination seem unlikely.



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