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.
Key takeaways
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.
