Using the Bradford-Hill criteria to assess causality in the association between CHADOX1 NCOV-19 vaccine and thrombotic immune thrombocytopenia

The Bradford-Hill criteria are accepted criteria for assessing causality of an association. Here, the criteria were applied to the association between the CHADOX1 NCOV-19 vaccine and vaccine induced thrombotic immune thrombocytopenia (VITT). All criteria for causation are met, with consistency, specificity, temporality and biological plausibility being very clearly met. Strength of association is met, but more data are required to establish the precise estimate of the association, as case ascertainment may be variable between countries, resulting in varied estimates of incidence rates from 25 to 0.5 per 100,000. (2, 4) The application of the modified Bradford-Hill criteria to VITT following CHADOX1 NCOV- vaccine strongly supports a causal relationship.