The clinical picture of moderate-to-severe thrombocytopenia and thrombotic complications at unusual sites beginning approximately 1 to 2 weeks after vaccination against SARS-CoV-2 with ChAdOx1 nCov-19 suggests a disorder that clinically resembles severe heparin-induced thrombocytopenia, a well-known prothrombotic disorder caused by platelet-activating antibodies that recognize multimolecular complexes between cationic PF4 and anionic heparin.6 However, unlike the usual situation in heparin-induced thrombocytopenia, these vaccinated patients did not receive any heparin to explain the subsequent occurrence of thrombosis and thrombocytopenia.

In recent years, it has been recognized that triggers other than heparin can cause a prothrombotic disorder that strongly resembles heparin-induced thrombocytopenia on both clinical and serologic grounds, including certain polyanionic drugs (e.g., pentosan polysulfate,7 antiangiogenic agent PI-88,8 and hypersulfated chondroitin sulfate8). Such a prothrombotic syndrome has also been observed in the absence of preceding exposure to any polyanionic medication, such as after both viral and bacterial infections9,10 and knee-replacement surgery.11,12 These various clinical scenarios with apparent nonpharmacologic triggers have been classified under the term autoimmune heparin-induced thrombocytopenia.13 Unlike patients with classic heparin-induced thrombocytopenia, patients with autoimmune heparin-induced thrombocytopenia have unusually severe thrombocytopenia, an increased frequency of disseminated intravascular coagulation, and atypical thrombotic events. Serum from these patients strongly activate platelets in the presence of heparin (0.1 to 1.0 IU per milliliter) but also in the absence of heparin (heparin-independent platelet activation). When these unusual antibodies are observed in patients who have thrombocytopenia without preceding heparin exposure, the term “spontaneous” heparin-induced thrombocytopenia syndrome13,14 has been used. Sometimes, patients in whom heparin-induced thrombocytopenia develops after exposure to heparin present with atypical clinical features, such as an onset of thrombocytopenia beginning several days after stopping heparin (delayed-onset heparin-induced thrombocytopenia15,16) or thrombocytopenia that persists for several weeks despite the discontinuation of heparin (persisting or refractory heparin-induced thrombocytopenia17,18). Serum from these patients also shows the phenomenon of heparin-independent platelet-activating properties.

These clinical features that resemble those of autoimmune heparin-induced thrombocytopenia were observed in the patients with vaccine-induced immune thrombotic thrombocytopenia. The serum usually showed strong reactivity on the PF4–heparin ELISA. Moreover, serum showed variable degrees of platelet activation in the presence of buffer that was in most cases greatly enhanced in the presence of PF4 (Figure 1A and 1B). More strikingly, most serum showed inhibition, rather than increased activation, in the presence of low-dose low-molecular-weight heparin (0.2 U per milliliter of anti–factor Xa). In addition, antibodies from two patients, which were affinity purified on either immobilized PF4 or immobilized PF4–heparin, strongly activated platelets but only in the presence of PF4. Enhancement of platelet activation by PF4 is also a feature of heparin-induced thrombocytopenia19,20 and has been used to enhance detection of platelet-activating antibodies in diagnostic testing for this adverse drug reaction.21 Whether these antibodies are autoantibodies against PF4 induced by the strong inflammatory stimulus of vaccination or antibodies induced by the vaccine that cross-react with PF4 and platelets requires further study.

Although we found enhanced reactivity of patient serum with platelets in the presence of ChAdOx1 nCov-19, this is likely to be an in vitro artifact. It is well known that adenovirus binds to platelets22 and causes platelet activation.22,23 Furthermore, the amount of adenovirus in a 500-microliter vaccine injection administered 1 or 2 weeks earlier would seem unlikely to contribute to subsequent platelet activation observed in these patients. However, interactions between the vaccine and platelets or between the vaccine and PF4 could play a role in pathogenesis. One possible trigger of these PF4-reactive antibodies could be free DNA in the vaccine. We have previously shown that DNA and RNA form multimolecular complexes with PF4, which bind antibodies from patients with heparin-induced thrombocytopenia and also induce antibodies against PF4–heparin in a murine model.24 Unfortunately, other Covid-19 vaccines were not available to us for testing.

Our findings have several important clinical implications. First, clinicians should be aware that in some patients, venous or arterial thrombosis can develop at unusual sites such as the brain or abdomen, which becomes clinically apparent approximately 5 to 20 days after vaccination. If such a reaction is accompanied by thrombocytopenia, it can represent an adverse effect of the preceding Covid-19 vaccination. To date, this reaction has been reported only with the ChAdOx1 nCov-19 vaccine, which has been used in approximately 25% of vaccine recipients in Germany and in 30% of those in Austria.

Second, ELISA to detect PF4–heparin antibodies in patients with heparin-induced thrombocytopenia is widely available and can be used to investigate patients for potential postvaccination thrombocytopenia or thrombosis associated with antibodies against PF4.25 A strongly positive ELISA result that is obtained in a patient who has not been recently exposed to heparin would be a striking abnormality.

Third, we have shown that these antibodies recognize PF4 and that the addition of PF4 greatly enhances their detectability in a platelet-activation assay. Since vaccination of millions of persons will be complicated by a background of thrombotic events unrelated to vaccination, a PF4-dependent ELISA or a PF4-enhanced platelet-activation assay may be used to confirm the diagnosis of vaccine-induced immune thrombotic thrombocytopenia through this novel mechanism of postvaccination formation of platelet-activating antibodies against PF4. Although treatment decisions such as administering intravenous immune globulin and starting anticoagulation do not need to await laboratory diagnosis, detection of these unusual platelet-activating antibodies will be highly relevant for case identification and future risk–benefit assessment of this and other vaccines.

Figure 2. Figure 2. Potential Diagnostic and Therapeutic Strategies for Management of Suspected Vaccine-Induced Immune Thrombotic Thrombocytopenia. Shown is a decision tree for the evaluation and treatment of patients who have symptoms of thrombocytopenia or thrombosis within 20 days after receiving the ChAdOx1 nCov-19 vaccine and who have had no heparin exposure. The diagnostic and therapeutic strategies in such patients differ from those in patients with autoimmune heparin-induced thrombocytopenia (HIT).13 DIC denotes disseminated intravascular coagulation, INR international normalized ratio, PF4 platelet factor 4, and PTT partial thromboplastin time.

Figure 2 shows a potential diagnostic and therapeutic strategy for managing this novel prothrombotic thrombocytopenic disorder. One consideration is to administer high-dose intravenous immune globulin to inhibit Fcγ receptor–mediated platelet activation. This recommendation parallels emerging experience in the treatment of severe autoimmune heparin-induced thrombocytopenia in which high-dose intravenous immune globulin has resulted in rapid increases in platelet count and de-escalation of hypercoagulability.12,26 We found that the addition of immune globulin in doses that are readily achieved clinically was effective in inhibiting platelet activation by patients’ antibodies. Clinician reluctance to start anticoagulation may be tempered by administering high-dose intravenous immune globulin to raise the platelet count, especially when a patient presents with severe thrombocytopenia and thrombosis, such as cerebral venous thrombosis.

Given the parallels with autoimmune heparin-induced thrombocytopenia, anticoagulant options should include nonheparin anticoagulants used for the management of heparin-induced thrombocytopenia,27 unless a functional test has excluded heparin-dependent enhancement of platelet activation. Finally, we suggest naming this novel entity vaccine-induced immune thrombotic thrombocytopenia (VITT) to avoid confusion with heparin-induced thrombocytopenia.