Vaccines and Rare Clotting Disorders: What's the Link?

  • Hamid Merchant

Press/Media: Expert Comment


Causal relation appears possible, but evidence still indicates it's extremely rare.

The possibility of a blood clot is not usually on the minds of most people when they roll up their sleeves for a vaccine, but lately that issue has been on the radar.

Recently, vaccinations with the AstraZeneca COVID-19 shot were put on hold in countries across Europe following reports of rare blood clots, low platelets, and hemorrhage. After formal review, the European Medicines Agency (EMA) and World Health Organization (WHO) both issued statements saying that the overall risk for clotting with this vaccine is no higher than in the general population, that the benefits outweigh the risks, and that vaccinations should continue. The American Society of Hematology has also said that the benefits of the AstraZeneca vaccine outweigh clotting risk.


Thrombotic events after CoViD-19 immunisation

Period25 Mar 2021

Media contributions


Media contributions

  • TitleVaccines and Rare Clotting Disorders: What's the Link?
    Degree of recognitionInternational
    Media name/outletMedPageToday
    Media typeWeb
    Country/TerritoryUnited States
    DescriptionCausal relation appears possible, but evidence still indicates it's extremely rare.One hypothesis is that the mRNA vaccines (Pfizer, Moderna) and those using adenoviruses (AstraZeneca, Johnson & Johnson) could induce synthesis of the COVID spike protein within platelets, which may then trigger autoimmune reactions against platelets, Hamid Merchant, PhD, of the University of Huddersfield in England, wrote in a letter to The BMJ."COVID genetic vaccines may have a direct role in spurring an autoimmune response against platelets that may clinically manifest in thrombocytopenia, hemorrhage, and blood clots. Clotting risks may be equally possible with all genetic COVID vaccines, and may not be limited only with the AstraZeneca/Oxford vaccine," he told MedPage Today.
    Producer/AuthorVeronica Hackethal
    PersonsHamid Merchant