Reports of rare blood clots occurring after COVID-19 vaccination have led some countries to limit the usage of AstraZeneca’s vaccine for certain categories of people, while other countries have paused the administration.
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An adverse effect following immunisation with the AstraZeneca jab is not merely blood clots in large vessels, but also a low platelet count.
The effect is similar to heparin-induced thrombocytopenia (HIT), where heparin, which is used to clear clots, actually causes blood clots and a fall in the platelet count.
The heparin combines with platelet factor 4 to form a complex. This immune complex induces an abnormal immune response, in which antibodies are generated against the complex.
This antibody-platelet factor 4-heparin immune complex binds to platelets and activates them. The platelets form clots everywhere. The low platelet count is because the platelets are used up.
A similar process seems to follow vaccination, except there is no heparin trigger. Scientists are still trying to establish a clinical definition for this adverse effect.
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