COVID-19: What is the blood clot risk from the Oxford-AstraZeneca vaccine and how does it compare?

The use of the Oxford-AstraZeneca vaccine in those under 30 is to be limited due to concerns over blood clots - but what exactly is the risk and how does it compare?

The UK's drugs regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), has said those aged 18-29 in the UK will be offered an alternative vaccine instead, where available, when invited to get a jab.

However, the MHRA and the EU's regulator - the European Medicines Agency (EMA) - have both stressed the overall benefits of the AstraZeneca vaccine in preventing COVID-19 outweigh the risks of side effects for the vast majority of people.

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What are the concerns?

According to the MHRA, up to 31 March, there have been 79 reports of blood clots accompanied by low blood platelet count in the UK, all in people who had their first dose.

Of these 79, 19 people have died, although it has not been established what the cause was in every case.

The 79 cases occurred in 51 women and 28 men, aged from 18 to 79.

Of the 19 who died, three were under the age of 30, the MHRA said.

Some 14 cases of the 19 were cerebral venous sinus thrombosis (CVST), a specific type of clot that prevents blood from draining from the brain.

The other five cases were thrombosis.

Dr June Raine, the chief executive of the MHRA, said: "From these reports the risk of this type of rare blood clot is about four people in a million who receive the vaccine."

This works out at a risk of one in 250,000, or 0.0004%.

Meanwhile, Dr Sabine Straus - the chair of the European Medicines Agency's safety committee - said that "frequency is difficult to assess" but added: "If you state the reporting rate is approximately one in 100,000 or even a little bit higher, that would reflect the risk".

What is the prevalence of clots in COVID-19 patients?

Sir Munir Pirmohamed, the chair of the Committee of Human Medicines, has stressed how it is "important to remember COVID-19 itself causes clotting and and it causes lower platelets".

He outlined how a recent paper had found:

• Pulmonary embolism, or clotting on the lungs, occurs in 7.8% of people who have COVID-19
• Deep vein thrombosis (DVT), or clotting in the legs, occurs in 11.2% of people who have COVID-19
• Of those who have COVID-19 and end up in an intensive therapy unit (ITU), 23% will have some form of clot
• COVID-19 causes strokes in 1.6% of people
• Up to 30% of people who have COVID-19 will get thrombocytopenia, which is lowering of the platelet count.

Sir Munir added: "That puts into context that the risk of clots and lower platelet is much higher with COVID-19 than these extremely rare events which are occurring with the vaccine."

What is the risk of clotting from the contraceptive pill?

In an article in the Lancet journal last year, Margaret McCartney - an NHS GP in Scotland - wrote of how the estimated incidence of a blood clot with the combined oral contraceptive pill is about five per 10,000 women per year.

This works out at a risk of one in 2,000 or 0.05%.

In the US, the National Blood Clot Alliance estimates that one in 1,000 women per year who are taking birth control pills will develop a clot, putting the risk at 0.1%.

What is the risk of blood clotting from flights?

According to the National Institute for Health and Care Excellence, the annual incidence of deep vein thrombosis (DVT) is estimated to be about one in 1,000 (or 0.1%).

However, it adds that the risk of developing DVT is increased two to threefold after flights of more than four hours.

The risk of travel-related DVT in healthy people works out at:

• One event per 106,667 flights, for flights lasting less than four hours
• One event per 4,656 flights, for flights lasting over four hours
• One event per 1,264 flights, for flights lasting over 16 hours

What is the risk of blood clotting from major surgery?

According to the National Blood Clot Alliance in the US, it is estimated that, even with preventative treatment, 3% of patients who undergo orthopaedic surgery (usually the reconstruction of joints) will develop DVT and 1.5% will develop pulmonary embolism (PE).

Without preventative treatment, up to 80% of orthopaedic surgical patients will develop DVT, and 10 to 20% will develop PE, it said.