In hundreds of patients worldwide, vaccination against
COVID-19 with adenovirus vector
vaccines (
ChAdOx1 nCoV-19;
Ad26.COV2.S) triggered platelet-activating anti-
platelet factor 4 (PF4)
antibodies inducing
vaccine-induced immune thrombotic
thrombocytopenia (VITT). In most VITT patients, platelet-activating anti-PF4-antibodies are transient and the disorder is discrete and non-recurring. However, in some patients platelet-activating
antibodies persist, associated with recurrent
thrombocytopenia and sometimes with relapse of
thrombosis despite therapeutic-dose anticoagulation. Anti-PF4
IgG antibodies measured by
enzyme-immunoassay (EIA) are usually detectable for longer than platelet-activating
antibodies in functional assays, but duration of detectability is highly assay-dependent. As more than 1 vaccination dose against
COVID-19 is required to achieve sufficient protection, at least 69 VITT patients have undergone subsequent vaccination with an
mRNA vaccine, with no relevant subsequent increase in anti-PF4 antibody titers,
thrombocytopenia, or thrombotic complications. Also, re-exposure to adenoviral vector-based
vaccines in 5 VITT patients was not associated with adverse reactions. Although data are limited, vaccination against
influenza also appears to be safe.
SARS-CoV-2 infection reported in 1 patient with preceding VITT did not influence anti-PF4 antibody levels. We discuss how these temporal characteristics of VITT provide insights into pathogenesis.