Pathologic Antibodies to Platelet Factor 4 after ChAdOx1 nCoV-19 Vaccination

Abstract

Background: The mainstay of control of the coronavirus disease 2019 (Covid-19) pandemic is vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Within a year, several vaccines have been developed and millions of doses delivered. Reporting of adverse events is a critical postmarketing activity.

Methods: We report findings in 23 patients who presented with thrombosis and thrombocytopenia 6 to 24 days after receiving the first dose of the ChAdOx1 nCoV-19 vaccine (AstraZeneca). On the basis of their clinical and laboratory features, we identify a novel underlying mechanism and address the therapeutic implications.

Results: In the absence of previous prothrombotic medical conditions, 22 patients presented with acute thrombocytopenia and thrombosis, primarily cerebral venous thrombosis, and 1 patient presented with isolated thrombocytopenia and a hemorrhagic phenotype. All the patients had low or normal fibrinogen levels and elevated d-dimer levels at presentation. No evidence of thrombophilia or causative precipitants was identified. Testing for antibodies to platelet factor 4 (PF4) was positive in 22 patients (with 1 equivocal result) and negative in 1 patient. On the basis of the pathophysiological features observed in these patients, we recommend that treatment with platelet transfusions be avoided because of the risk of progression in thrombotic symptoms and that the administration of a nonheparin anticoagulant agent and intravenous immune globulin be considered for the first occurrence of these symptoms.

Conclusions: Vaccination against SARS-CoV-2 remains critical for control of the Covid-19 pandemic. A pathogenic PF4-dependent syndrome, unrelated to the use of heparin therapy, can occur after the administration of the ChAdOx1 nCoV-19 vaccine. Rapid identification of this rare syndrome is important because of the therapeutic implications.

Figure 1. Flow Cytometric Analysis of Results of a Functional HIT Assay in a Control Patient and a Patient in the Study.

Shown is the flow cytometric analysis of results of a functional heparin-induced thrombocytopenia (HIT) assay (HITAlert, Diapharma) performed on a serum sample from a control patient with a confirmed diagnosis of HIT (Panel A) and on a serum sample from a patient included in the study (Panel B). Platelets from a volunteer donor with group O blood are incubated in five tubes containing the following substances: calcium ionophore, heparin (0.3 U per milliliter), patient serum, patient serum plus heparin (0.3 U per milliliter), and patient serum plus an excess of heparin (100 U per milliliter). In the analysis of the control sample (Panel A), donor platelets show reactivity in the presence of calcium ionophore, with 98% of platelets positive for both anti–CD41–PE and anti–annexin V–FITC conjugated antibodies (data are shown in the upper right quadrant of each plot); reduced reactivity in the presence of heparin (0.3 U per milliliter), with only 0.6% of CD41-positive platelets also positive for annexin V; reduced reactivity in the presence of patient serum, with only 0.5% of platelets activated; reactivity in the presence of patient serum plus heparin (0.3 U per milliliter), with 41% of platelets activated; and reduced reactivity in the presence of patient serum plus an excess of heparin (100 U per milliliter), with 0.4% of platelets activated, as compared with 41% with patient serum plus heparin in a physiologic dose. In the analysis of the study sample (Panel B), donor platelets show reactivity in the presence of patient serum, with 55% of platelets activated; reactivity in the presence of patient serum plus heparin (0.3 U per milliliter), with 37% of platelets activated; and reduced reactivity in the presence of patient serum plus an excess of heparin (100 U per milliliter), with 1% of platelets activated. FITC denotes fluorescein isothiocyanate, and PE phycoerythrin.

Figure 2. Suggested Algorithm for Testing and Treatment of Patients Presenting with Thrombosis and Thrombocytopenia 5 to 30 Days after Vaccination.

The HemosIL AcuStar HIT IgG assay is not recommended for the evaluation of suspected vaccine-induced thrombosis and thrombocytopenia. ELISA denotes enzyme-linked immunosorbent assay, FEU fibrinogen-equivalent units, HIT heparin-induced thrombosis, and IVIG intravenous immune globulin.