LAUSR

Vaccine-induced Thrombotic Thrombocytopenia (VITT)is triggered by vaccination against COVID-19 with adenovirus vectorvaccines (ChAdOx1 nCoV-19; Ad26.COV2-S). In this observational study, we followed VITT patients for changes in their reactivity ofplatelet-activating anti-platelet factor 4 (PF4) IgG antibodiesby an anti-PF4/heparin IgG enzyme immunoassay (EIA) and a functional test for PF4-dependent, platelet-activating antibodies, and new thrombotic complications. Sixty-fiveVITT patients (41 females; median 51 years; range 18-80 years) were followed for 25 weeks (median; range,3-36weeks). In 48/65 patients (73.8%; CI, 62.0%-83.0%) the functional assay became negative. Median time to negative functional test result was 15.5 weeks (range 5-28 weeks). In parallel, EIA optical density (OD) valuesdecreased from median 3.12to 1.52(p<0.0001), but sero-reversion to a negative result was seen in only 14 (21.5%) patients. Five patients (7.5%) showed persistent platelet-activating antibodies and high EIA ODs for >11 weeks. None of the 29 VITT patients who received a second vaccination dose with an mRNA COVID-19 vaccinedeveloped new thromboses or relevant increase in anti-PF4/heparin IgG EIA OD, regardless whether PF4-dependent platelet-activating antibodies were still present.PF4-dependent platelet-activating antibodies are transient in most patients with VITT. VITT patients can safely receive a second COVID-19 mRNA-vaccine shot.