Is there a link between vaccination for COVID-19 and developing blood clots?

Written by: Sarah Lambert

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What is the risk of developing a blood clot
following Vaxzevria (Oxford AstraZeneca)
vaccination for COVID-19?

 

EMA: On 7 April 2021, the EMA stated that unusual blood clots in the brain (cerebral venous sinus thrombosis) with low blood platelets (thrombocytopenia) should be listed as very rare side effects of the Oxford AstraZeneca COVID-19 vaccine Vaxzevria. The EMA also said that there may be a possibility of these blood clots occurring within two weeks of vaccination, especially in women under 60 years of age.1

UK MHRA: On the same day, the UK MHRA issued new advice, stating that there is a possible link between the AstraZeneca vaccine and extremely rare blood clots. The MHRA found 79 cases in the UK, including 19 deaths, in 20 million vaccinated, giving an overall risk of clots in 1 in 250,000 vaccinations.2

The MHRA found an overall risk of clots in 1 in 250,000 vaccinations

 

What is the latest advice from the EMA and MRHA regarding Vaxzevria vaccinations?

EMA: 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.1

UK MHRA: Giving the AstraZeneca vaccine to people of any age who are at higher risk of blood clots because of a medical condition should be considered only if the benefit of protection from COVID-19 infection outweighs potential risks of the vaccination.2 The UK Joint Committee on Vaccination and Immunisation currently advises that it is preferable for adults aged <30 years without underlying health conditions to be offered an alternative vaccine, if available, although people may make an informed choice to receive the AstraZeneca vaccine for earlier protection.3

 

What is the risk of developing a blood clot following Comirnaty (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccination for COVID-19?

A study published on 14 April 2021 looking at over 500,000 COVID-19 cases and 490,000 people who were given one of the mRNA vaccines (Comirnaty [Pfizer-BioNTech] or Moderna COVID-19 Vaccine) showed that the risk of developing a blood clot after COVID-19 infection is approximately 8–10 times higher than reported for the vaccines, and about 100–fold higher compared with the general population.4 This study also showed that the risk of developing a brain blood clot following the AstraZeneca vaccine was lower than that following infection with COVID-19.4

The risk of developing a brain blood clot following the AstraZeneca vaccine was lower than that following infection with COVID-19

mRNA, Messenger Ribonucleic Acid; NRV, non-replicating viral vector

 

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What happens next?

Both the EMA and MHRA will continue looking at the evidence of a link between COVID-19 vaccination and development of blood clots.1,5 As of 12 March 2021, nine EU countries had suspended use of the AstraZeneca vaccine,6 although Germany, Italy, Spain and France are among those that have since resumed the AstraZeneca vaccine roll-out.7

The EMA is also looking at very rare cases of blood clots that occurred in the US following the use of COVID-19 Vaccine Janssen, with a recommendation expected late April.5 The rollout of the Janssen vaccine had been paused in the US while the FDA and CDC reviewed reports of 6 blood clot cases, but this was lifted and the rollout resumed on the 23 April 2021.5,8

 

EMA, European Medicines Agency; UK MHRA, United Kingdom Medicines and Healthcare products Regulatory Agency

 

References
  1. European Medicines Agency press release 7 April 2021. Available at: https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-finds-possible-link-very-rare-cases-unusual-blood-clots-low-blood [Accessed April 2021].
  2. Medicines & Healthcare products Regulatory Agency press release 7 April 2021. Available at: https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots [Accessed April 2021].
  3. Joint Committee on Vaccination and Immunisation statement on use of the AstraZeneca COVID-19 vaccine: 7 April 2021. Available at: https://www.gov.uk/government/publications/use-of-the-astrazeneca-covid-19-vaccine-jcvi-statement/jcvi-statement-on-use-of-the-astrazeneca-covid-19-vaccine-7-april-2021 [Accessed April 2021].
  4. Taquet M, et al. OSF. Available at: https://osf.io/a9jdq/ [Accessed April 2021].
  5. European Medicines Agency press release 14 April 2021. Available at: https://www.ema.europa.eu/en/news/covid-19-vaccine-janssen-assessment-very-rare-cases-unusual-blood-clots-low-platelets-continues [Accessed April 2021].
  6. Wise J. BMJ 2021;372:n699.
  7. Countries resume use of AstraZeneca vaccine, while some lose confidence. 22 March 2021. Available at: https://www.reuters.com/article/us-health-coronavirus-astrazeneca-vaccin-idUSKBN2BE0V6 [Accessed April 2021].
  8. US Food and Drug Agency news release 23 April 2021. Available at: https://www.fda.gov/news-events/press-announcements/fda-and-cdc-lift-recommended-pause-johnson-johnson-janssen-covid-19-vaccine-use-following-thorough [Accessed April 2021].

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