Possible Link Between AstraZeneca’s COVID-19 Vaccine and Blood Clots: Under 30s to be given Pfizer’s or Moderna’s mRNA Vaccine

MHRA, UK regulator has issued an advisory against the use of AstraZeneca vaccine as it has been shown to induce the formation of blood clots along with thrombocytopenia in rare cases (4 events in a million). However, in people where this condition does not occur, these can get the second dose administered when invited. 

Although blood clot formation along with thrombocytopenia is a rare event, it has shown an increasing tendency with decreasing age, with higher incidence found in younger population which are less than 30 years of age. However, contrastingly, a high risk of severe disease associated with COVID-19 increases with age, with the youngest population at lowest risk. Although the clotting event is extremely rare, still its sheer presence puts a question mark on the use of AstraZeneca vaccine despite its efficacy for the human population for COVID-19 disease prevention. In the wake of these events, JCVI currently advises that it is preferable for adults aged <30 years without underlying health conditions that put them at higher risk of severe COVID-19 disease. 

In any case, as more data comes out from vaccine administration, it will become increasingly clear whether the benefits potentially outweighing the risks of taking the vaccine with regard to blood clot formation. 

Keeping the rare blood clotting event in mind, it has been proposed that the population under the age of 30 being given the Pfizer/Moderna vaccine instead of the AstraZeneca one. 

On the other hand, the company AstraZeneca says that 37 blood clots have been reported out of more than 17 million people vaccinated in the EU and Britain. This number is much lower than would be expected to occur naturally in a general population of this size and is similar across other licensed covid-19 vaccines. In addition, Phil Bryan, vaccines safety lead at the UK Medicines and Healthcare Products Regulatory Agency, said, “Blood clots can occur naturally and are not uncommon and the number of blood clots that have been reported after vaccine administration is not greater than the number that would have occurred naturally in the vaccinated population. 



  1. MHRA 2021. Press Release – MHRA issues new advice, concluding a possible link between COVID-19 Vaccine AstraZeneca and extremely rare, unlikely to occur blood clots. Available online at 
  1. JCVI statement on use of the AstraZeneca COVID-19 vaccine: 7 April 2021 Published 7 April 2021. Available online at 
  1. Vogel G and Kupferschmidt K. 2021. Side effect worry grows for AstraZeneca vaccine. Science 02 Apr 2021: Vol. 372, Issue 6537, pp. 14-15 
  1. Covid-19: WHO says rollout of AstraZeneca vaccine should continue, as Europe divides over safety. BMJ 2021; 372 DOI: (Published 16 March 2021) 
  1. Covid-19: European countries suspend use of Oxford-AstraZeneca vaccine after reports of blood clots. BMJ 2021; 372 DOI: (Published 11 March 2021) 
  1. Thromboembolism and the Oxford–AstraZeneca COVID-19 vaccine: side-effect or coincidence? Lancet. 2021 Mar 30  DOI: 


Rajeev Soni
Rajeev Soni
Dr. Rajeev Soni (ORCID ID : 0000-0001-7126-5864) has a Ph.D. in Biotechnology from the University of Cambridge, UK and has 25 years of experience working across the globe in various institutes and multinationals such as The Scripps Research Institute, Novartis, Novozymes, Ranbaxy, Biocon, Biomerieux and as a principal investigator with US Naval Research Lab in drug discovery, molecular diagnostics, protein expression, biologic manufacturing and business development.

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