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It really is a matter of statistics. If condition A occurs without an underlying cause in the general population at the frequency of 1 in 100,000 which is rare, and occurs with the same frequency or even less in those who had the Astra Zeneca vaccine than you cannot establish a direct link with the two events. If however 5 out of 100,000 who had the condition then have complication A then there may be a link. The same say would apply if several people who have had the vaccine were run over by a bus the next day, you wouldn’t ascribe this to the vaccine.
This website gives some of the underlying causes that predispose to CVST
Amongst them is a group of rare disorders of the blood clotting mechanism and also autoimmune disorders. Some of these disorders are inherited but not everyone who inherits them will get a clot, other precipitating factors can then contribute to the clotting, such as post surgery or during pregnancy, or associated with infection or cancer, where there is a natural tendency also for the blood to clot more easily.