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April 07, 2021
1 min read

European regulators say blood clots ‘very rare side effect’ of AstraZeneca vaccine

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The European Medicines Agency said Wednesday that “unusual blood clots with low blood platelets should be listed as [a] very rare side effect” of AstraZeneca’s COVID-19 vaccine.

“The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects,” the agency said in a news release.

Astrazeneca stock image
The European Medicines Agency said that blood clots should be listed as a ‘very rare side effect’ of AstraZeneca’s COVID-19 vaccine.
Source: Adobe Stock

Last month, some European countries temporarily stopped administering the vaccine due to reports of blood clots in people who received it. The European Medicines Agency (EMA) later declared the vaccine safe and said an investigation revealed no link between the vaccine and thromboembolism or blood clots.

In reaching its latest conclusion, EMA’s safety committee “took into consideration all currently available evidence, including the advice from an ad hoc expert group,” the agency said.

“EMA is reminding health care professionals and people receiving the vaccine to remain aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within 2 weeks of vaccination. So far, most of the cases reported have occurred in women under 60 years of age within 2 weeks of vaccination,” the agency said. “Based on the currently available evidence, specific risk factors have not been confirmed.”

The safety committee said clots occurred in veins in the brain and abdomen, and in arteries. The committee reviewed 62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis that occurred as of March 22 in the European Union, 18 of which were fatal.

The United Kingdom’s Medicines and Healthcare products Regulatory Agency said it conducted an updated review of cases and found a “slightly higher” risk for these events than in a previous review, but also noted that they were “extremely rare.”

AstraZeneca said regulators have requested that the company update the vaccine’s label in the U.K. and European Union.

“Overall, both of these reviews reaffirmed the vaccine offers a high-level of protection against all severities of COVID-19 and that these benefits continue to far outweigh the risks,” the company said in the release.

According to the EMA, around 34 million people have received the AstraZeneca vaccine in the U.K. and European Economic Area.

A potential explanation for the clotting and low blood platelets may be an immune response, leading to a condition similar to heparin-induced thrombocytopenia, the EMA said. It said the safety committee has requested more research to better understand the instances of clotting.


AstraZeneca. Update following MHRA and EMA decisions on AstraZeneca’s COVID-19 vaccine. Accessed April 7, 2021.

MHRA. MHRA issues new advice, concluding a possible link between COVID-19 Vaccine AstraZeneca and extremely rare, unlikely to occur blood clots. Accessed April 7, 2021.

Press Release