Coronavirus: what is the AstraZeneca blood-clot risk?

The use of the vaccine in under-30s will be limited owing to concerns over blood clots

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Related:  Europe divided on AstraZeneca vaccine after regulators find link to rare blood clots

The UK’s decision this week to stop giving the Oxford-AstraZeneca vaccine to under-30s underlined the risks from medications and medical procedures.

It came after health authorities in at least six European countries restricted the shot to those above a certain age, varying by country from 55 to 70 years.

Regulators emphasise that the dangers of cerebral venous sinus thrombosis (CVST) blood clots in the brain are small and, for most, far outweighed by the threats from Covid-19.

If the risk is something like one in 250,000, it's a small risk, but obviously a tragedy for those involved

Up to the end of March in the UK there were thought to have been 79 cases of CVST from 20 million doses of the Oxford-AstraZeneca vaccine, 19 of which were fatal.

This gives a blood clot risk of about one in 250,000 and a risk of death of about one in a million – about the same, reports said, as a 400-kilometre car journey.

“If the risk is something like one in 250,000, it’s a small risk, but obviously a tragedy for those involved,” said Prof David Taylor, professor emeritus of pharmaceutical and public health policy at University College London. But how does it compare with the risk of having surgery?


Risks from surgery are balanced against risks from not having an operation, which for some patients can be very high.

Surgery in which the heart has to be stopped is more dangerous, but even operations such as tonsil removal can prove fatal. A 2019 study found that, globally, 4.2 million people each year die within 30 days of surgery.

Abdominal gastrointestinal surgery to combat obesity carries a 1 per cent risk of death, according to the UK's National Health Service, with fatalities normally the result of a blood clot in the lungs or a gastrointestinal leak.


"Painkillers kill about 2,000 people a year in the UK alone if you take the cardiovascular risk and everything else into account," Prof Taylor said.

That figure is from a report by Swiss and British researchers highlighting potential dangers from medications such as ibuprofen and aspirin. Such non-steroidal anti-inflammatory drugs can cause complications, including gastrointestinal bleeding.

Work from 2014 found these drugs cause an 80 per cent increase in blood clots in the veins, which can travel to the heart or lungs and may be fatal.


These spark an allergic reaction in about one in 15 people taking them, the UK’s National Health Service said, and while such reactions are usually mild, anaphylactic shock can prove fatal.

Antibiotics cause about 40 per cent of fatal drug-induced anaphylaxis, with substances given to help medical diagnosis (such as radiocontrast agents, which show up structures on X-rays) and anaesthetics causing much of the rest.

Between 1999 and 2010, about 1,445 people died in the US because of anaphylaxis caused by a drug, and rates over that period doubled.

Anaphylaxis is a severe allergic reaction to venom, food, or medication

The contraceptive pill

Oral contraceptives taken by women to prevent pregnancy are associated with an increase in potentially fatal blood clots.

The National Blood Clot Alliance in the US estimates that among women on birth control pills, about 1 in 1,000 a year will suffer a blood clot. Figures from Scotland put the figure at about half this. The BBC worked out that the contraceptive pill resulted in a sixfold increase in CVST blood clots, while a 2015 study suggested there was a 7.59-fold increase. The coronavirus vaccine (based on BBC calculations using data from European regulators) causes a ninefold increase.