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AstraZeneca COVID-19 Vaccine Latest: Risk Of Blood Clots Appears To Be Extremely Small


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist


Image credit: Mike Mareen/

The benefits of receiving the AstraZeneca COVID-19 vaccine very favorably outweigh the potential risks for the vast majority of people. That’s the latest conclusion from the UK health regulator (MHRA) on the ongoing debate surrounding unusual blood clots linked to the AstraZeneca vaccine.

The MHRA said the unusual blood clots linked to the vaccine are extremely rare — in the UK, 20.2 million doses of the AstraZeneca COVID-19 vaccine had been given and 79 people have reported blood clotting. The MHRA also stressed the evidence of a link between blood clots with the AstraZeneca vaccine is “getting firmer” but more work is still needed.


The announcements follow an unfolding saga over the past month in which a handful of countries suspended the use of the AstraZeneca COVID-19 vaccine over a suspected link to blood clots.

At a media conference on Wednesday, the MHRA announced the findings of their extensive review on the issue, concluding that the blood clots have an extremely rare rate of occurrence. They added that healthy adults in the UK aged between 18 and 29 years old should be offered an alternative vaccine to the AstraZeneca shot if available, a suggestion it's only making out of the "utmost caution."


The UK’s decision regarding under-30s may appear a bit daunting, but it’s a question of potential risk versus potential benefits (as you can see in the graph below). The blood clots appear to be occurring in fractionally more young people than expected. Nevertheless, under-30s have a low risk of getting a blood clot following the vaccine and a relatively low risk of being hospitalized with COVID-19. Given this subtle risk-benefit balance, the regulators believe it's advisable for under-30s to receive an alternative vaccine if possible.

“We are not advising a stop to any vaccination for any individual in any age group. We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution, rather than because we have any serious safety concerns,” Wei Shen Lim, COVID-19 Chair for UK's Joint Committee on Vaccination and Immunisation, said at the briefing. 


Meanwhile, the safety committee of the European Medicine Agency (EMA) also announced today that unusual blood clots with low blood platelets should be listed as "very rare side effects".

The occurrence of blood clots, however, appears to be extremely rare. Some 20.2 million doses of the COVID-19 Vaccine AstraZeneca had been given in the UK as of March 31 and at least 79 people have experienced blood clotting. This means the overall risk of these blood clots is approximately 4-in-1,000,000 people who receive the vaccine. Up to 44 of the 79 cases were of a specific type of blood clot in the brain, known as cerebral venous sinus thrombosis (CVST) occurring together with low levels of platelets in the blood, while 35 of the cases were of thrombosis in other major veins. Tragically, 19 people have died out of the 79 cases – 13 females and 6 males.

"No effective medicine or vaccine is without risk. We continually monitor safety during widespread use of any vaccine. This is to ensure vaccines are performing as expected, to identify any new side effects that may arise, and to ensure the benefits continue to outweigh the risks," said Dr June Raine, MHRA Chief Executive.

"It is still vitally important that people come forward for their vaccination when invited to do so," Raine added. 


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