The COVID-19 pandemic has accelerated the development of SARS-CoV-2 vaccines. Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is an uncommon but potentially fatal consequence of mostly adenoviral-based vaccinations that is caused by the existence of antibodies to a PF4/polyanion neoepitope and evaluated by enzyme-linked immunosorbent assays. 

Serial anti–PF4/polyanion antibody, platelet, and D-dimer assays in a large patient cohort were shown, as well as their relationship to relapse. Overall, 51% of patients examined with the Stago test had persistently positive anti–PF4/polyanion levels 100 days after diagnosis, whereas 94% of patients evaluated with the Immucor assay remained positive. The median duration of PF4 test positivity is 87 days, with 72% of patients remaining positive following a 105-day follow-up. 

In the acute context, plasma exchange appeared to lower anti–PF4/polyanion levels and improve platelet counts more quickly than conventional therapy. In the investigation, the recurrence rate was 12.6%, with all relapsed patients showing consistently positive PF4 antibodies and declining platelet counts. Only one patient’s thrombosis had spread. Despite the preservation of PF4 antibodies in 72% of patients, the risk of relapse was modest and did not appear to result in the severe clinical picture seen at index presentation. The cohort’s monitoring of the patients was ongoing and would help define the natural history of this unique illness.