Potential mechanisms of vaccine-induced thrombosis.

Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a rare syndrome characterized by high-titer anti-platelet factor 4 (PF4) antibodies, thrombocytopenia and arterial and venous thrombosis in unusual sites, as cerebral venous sinuses and splanchnic veins. VITT has been described to occur almost exclusively after administration of ChAdOx1 nCoV-19 and Ad26.COV2.S adenovirus vector- based COVID-19 vaccines. Clinical and laboratory features of VITT resemble those of heparin-induced thrombocytopenia (HIT). It has been hypothesized that negatively charged polyadenylated hexone proteins of the AdV vectors could act as heparin to induce the conformational changes of PF4 molecule that lead to the formation of anti-PF4/polyanion antibodies. The anti-PF4 immune response in VITT is fostered by the presence of a proinflammatory milieu, elicited by some impurities found in ChAdOx1 nCoV-19 vaccine, as well as by soluble spike protein resulting from alternative splice events. Anti-PF4 antibodies bind PF4, forming immune complexes which activate platelets, monocytes and granulocytes, resulting in the VITT's immunothrombosis. The reason why only a tiny minority of patents receiving AdV-based COVID-19 vaccines develop VITT is still unknown. It has been hypothesized that individual intrinsic factors, either acquired (i.e., pre-priming of B cells to produce anti-PF4 antibodies by previous contacts with bacteria or viruses) or inherited (i.e., differences in platelet T-cell ubiquitin ligand-2 [TULA-2] expression) can predispose a few subjects to develop VITT. A better knowledge of the mechanistic basis of VITT is essential to improve the safety and the effectiveness of future vaccines and gene therapies using adenovirus vectors.

Vaccine-induced immune thrombotic thrombocytopenia post dose 2 ChAdOx1 nCoV19 vaccination: Less severe but remains a problem.

BACKGROUND: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but established complication of 1st dose ChAdOx1 nCoV19 vaccination (AZD1222), however this complication after dose 2 remains controversial. OBJECTIVES: To describe the clinicopathological features of confirmed cases of VITT post dose 2 AZD1222 vaccination in Australia, and to compare this cohort to confirmed cases of VITT post 1st dose. METHODS: Sequential cases of clinically suspected VITT (thrombocytopenia, D-Dimer > 5x upper limit normal and thrombosis) within 4-42 days of dose 2 AZD1222 referred to Australia's centralised testing centre underwent platelet activation confirmatory testing in keeping with the national diagnostic algorithm. Final classification was assigned after adjudication by an expert advisory committee. Descriptive statistics were performed on this cohort and comparative analyses carried out on confirmed cases of VITT after 1st and 2nd dose AZD1222. RESULTS: Of 62 patients referred, 15 demonstrated presence of antibody mediated platelet activation consistent with VITT after dose 2 AZD1222. Four were immunoassay positive. Median time to presentation was 13 days (range 1-53) platelet count 116x10^9/L (range 63-139) and D-dimer elevation 14.5xULN (IQR 11, 26). Two fatalities occurred. In each, the dosing interval was less than 30 days. In comparison to 1st dose, dose 2 cases were more likely to be male (OR 4.6, 95% CI 1.3-15.8, p = 0.03), present with higher platelet counts (p = 0.05), lower D-Dimer (p = 01) and less likely to have unusual site thromboses (OR 0.14, 95% CI 0.04-0.28, p = 0.02). CONCLUSIONS: VITT is a complication of dose 2 AZD1222 vaccination. Whilst clinicopathological features are less severe, fatalities occurred in patients with concomitant factors.

Microsurgical complication associated with vaccine-induced immune thrombotic thrombocytopenia (VITT): A case report.

RATIONALE: The use of ChAdOx1 nCoV-19 (Astra Zeneca) vaccine has proven beneficial, but in a limited number of the general population, it was found to be associated with vaccine-induced immune thrombotic thrombocytopenia (VITT). However, there have been no reports of this complication occurring in a microsurgical free tissue transfer. PATIENT CONCERNS: A 49-year-old man developed an acute myocardial infarction 3 weeks after receiving his first dose of ChAdOx1 nCoV-19 in June 2021. Three months later, he presented with right third toe wet gangrene with extension into the plantar foot nine days after receiving his second dose of ChAdOx1 nCoV-19 vaccine. DIAGNOSIS: Based on recent exposure to vaccination, the timing of inoculation before the development of his symptoms, and serology tests (platelet, D-dimer, and anti-PF4 antibodies), the patient was diagnosed with VITT. INTERVENTIONS: Fasciectomy and sequestrectomy were performed for wound bed preparation. Limb salvage was done using free vastus lateralis muscle flap and skin graft for reconstruction. OUTCOME: The flap was complicated by persistent microthrombi leading to superficial necrosis without vascular pedicle compromise. Repeated debridement of the superficial necrosis was done. Three months after the development of VITT, no further new superficial necrosis was seen. A well-contoured flap was seen 5 months after the initial surgery. LESSONS: We believe this is the first case describing microthrombi in the free flap due to VITT after microsurgical reconstruction. Patients and surgeons should be advised of this possible risk when contemplating microsurgery once VITT has developed after ChAdOx1 nCoV-19 administration.

Use of thrombopoietin receptor agonists in adults with immune thrombocytopenia: a systematic review and Central European expert consensus.

There are currently three thrombopoietin receptor agonists (TPO-RAs) approved in Europe for treating patients with immune thrombocytopenia (ITP): romiplostim (Nplate®), eltrombopag (Revolade®), and avatrombopag (Doptelet®). However, comparative clinical data between these TPO-RAs are limited. Therefore, the purpose of this study was to perform a literature review and seek expert opinion on the relevance and strength of the evidence concerning the use of TPO-RAs in adults with ITP. A systematic search was conducted in PubMed and Embase within the last 10 years and until June 20, 2022. A total of 478 unique articles were retrieved and reviewed for relevance. The expert consensus panel comprised ITP senior hematologists from eight countries across Central Europe. The modified Delphi method, consisting of two survey rounds, a teleconference and email correspondence, was used to reach consensus. Forty articles met the relevancy criteria and are included as supporting evidence, including five meta-analyses analyzing all three European-licensed TPO-RAs and comprising a total of 31 unique randomized controlled trials (RCTs). Consensus was reached on seven statements for the second-line use of TPO-RAs in the management of adult ITP patients. In addition, the expert panel discussed TPO-RA treatment in chronic ITP patients with mild/moderate COVID-19 and ITP patients in the first-line setting but failed to reach consensus. This work will facilitate informed decision-making for healthcare providers treating adult ITP patients with TPO-RAs. However, further studies are needed on the use of TPO-RAs in the first-line setting and specific patient populations.

Vaccine-induced immune thrombotic thrombocytopenia.

Within the first months of the COVID-19 vaccination campaign, previously healthy recipients who developed severe thrombosis (often cerebral and/or splanchnic vasculature) and thrombocytopenia typically after adenoviral vector-based vaccination were identified. Similarities between this syndrome, vaccine-induced immune thrombotic thrombocytopenia (VITT), and heparin-induced thrombocytopenia prompted recognition of the role of antiplatelet factor 4 (PF4) antibodies and management strategies based on IV immunoglobulin and nonheparin anticoagulants, which improved outcome. We update current understanding of VITT and potential involvement of anti-PF4 antibodies in thrombotic disorders.

Diagnostic Performance of a Particle Gel Immunoassay in Vaccine-Induced Immune Thrombotic Thrombocytopenia.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but serious complication of adenoviral vector-based COVID-19 vaccines. Similar to heparin-induced thrombocytopenia (HIT), antibodies reacting to platelet factor 4 (PF4) are responsible for platelet activation in VITT. The diagnosis of VITT includes the detection of anti-PF4 antibodies. Particle gel immunoassay (PaGIA) is one of the rapid immunoassays that is commonly used in the diagnosis of HIT to detect anti-PF4 antibodies. The aim of this study was to investigate the diagnostic performance of PaGIA in patients suspected of VITT. In this retrospective, single-center study, the correlation between PaGIA, enzyme immunoassay (EIA), and modified heparin-induced platelet aggregation assay (HIPA) in patients with findings suggestive of VITT was investigated. A commercially available PF4 rapid immunoassay (ID PaGIA H/PF4, Bio-Rad-DiaMed GmbH, Switzerland) and an anti-PF4/heparin EIA (ZYMUTEST HIA IgG, Hyphen Biomed) were used according to manufacturer's instructions. Modified HIPA was accepted as the gold standard test. Between March 8 and November 19, 2021, a total of 34 samples from clinically well-characterized patients (14 males, 20 females, mean age: 48.2 ± 18.2 years) were analyzed with PaGIA, EIA, and modified HIPA. VITT was diagnosed in 15 patients. Sensitivity and specificity of PaGIA were 54 and 67%, respectively. Anti-PF4/heparin optical density values were not significantly different between PaGIA positive and negative samples (p = 0.586). The sensitivity and specificity of EIA, on the other hand, were 87 and 100%, respectively. In conclusion, PaGIA is not reliable in the diagnosis of VITT because of its low sensitivity and specificity.

Increased incidence of immune thrombocytopenia (ITP) in 2021 correlating with the ongoing vaccination campaign against COVID-19 in a tertiary center - A monocentric analysis.

Immune thrombocytopenia (ITP) was reported as a rare complication of COVID-19 vaccines. We conducted a retrospective single-center analysis of all ITP cases detected in 2021 and compared the quantity with the pre-vaccination years, from 2018 to 2020. In 2021, a two-fold increase in ITP cases was identified compared to previous years; 11 of 40 cases (27.5%) were considered COVID-19-vaccine related. Our study highlights an increase in ITP cases at our institution, probably related to COVID-19 vaccinations. Further studies are needed to investigate this finding globally.

Vaccine-associated thrombocytopenia.

Vaccination is the most cost-effective means of preventing and even eliminating infectious diseases. However, adverse reactions after vaccination are inevitable. In addition to common vaccine-related adverse reactions, some rare but serious adverse reactions have been reported, including secondary immune thrombocytopenia (ITP). The measles-mumps-rubella (MMR) vaccine is currently the only vaccine for which a cause-effect relationship with immune thrombocytopenia has been demonstrated with an incidence of approximately 0.087-4 per 100,000 doses, and the complication is mostly observed in children. In addition, thrombocytopenia can be induced by coronavirus disease 2019 (COVID-19) vaccines following COVID-19 vaccination primarily occurs within a few weeks post-vaccination. The condition mostly occurs in elderly individuals with no sex differences. Its incidence is approximately 0.80 to 11.3 per million doses. Some patients have previously suffered from chronic ITP likely to develop exacerbation of ITP after COVID-19 vaccines, especially those who have undergone splenectomy or are being treated with >5 medications. Based on clinical practice, first-line treatments for vaccine-associated thrombocytopenia are essentially limited to those used for primary ITP, including glucocorticoids and intravenous immunoglobulin (IVIg).

Autoimmune haemolytic anaemia and immune thrombocytopenia following SARS-CoV-2 and non-SARS-CoV-2 vaccination: 32 Years of passive surveillance data.

Autoimmune haemolytic anaemia (AIHA) and immune thrombocytopenia (ITP) are two uncommon haematologic autoimmune conditions that can rarely arise secondary to vaccination. Prior studies using the US Centers for Disease Control's (CDC) Vaccine Adverse Event Reporting System (VAERS) have demonstrated this infrequency, but contemporary data as well as comparison with current information regarding SARS-CoV-2 vaccination has not been assessed. In this study, we reviewed VAERS database reports from 1990 to 2022 to characterize the incidence and clinical and laboratory findings of non-SARS-CoV-2-associated AIHA and ITP and SARS-CoV-2 vaccine-associated AIHA and ITP. We discovered a total of 863 AIHA and ITP reports following vaccination with 15 non-SARS-CoV-2 and four SARS-CoV-2 vaccines submitted to the CDC VAERS database. AIHA and ITP reporting was low for both groups, with a large proportion excluded due to a lack of clinical details. ITP was reported the most frequently in both groups and was significantly more common with measles-mumps-rubella (MMR) vaccination (p < 0.001) in the non-SARS-CoV-2 group. AIHA and ITP cases were higher in the SARS-CoV-2 vaccine group, though ultimately still very infrequent. Autoimmune haematologic disease is vanishingly rare after immunization and rates are lower than in the general population according to passive reporting.

Analysis of hematologic adverse events reported to a national surveillance system following COVID-19 bivalent booster vaccination.

Hematologic complications, including vaccine-induced immune thrombotic thrombocytopenia (VITT), immune thrombocytopenia (ITP), and autoimmune hemolytic anemia (AIHA), have been associated with the original severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines. However, on August 31, 2022, new formulations of the Pfizer-BioNTech and Moderna vaccines were approved for use without clinical trial testing. Thus, any potential adverse hematologic effects with these new vaccines remain unknown. We queried the US Centers for Disease Control Vaccine Adverse Event Reporting System (VAERS), a national surveillance database, through February 3, 2023, all reported hematologic adverse events that occurred within 42 days of administration of either the Pfizer-BioNTech or Moderna Bivalent COVID-19 Booster vaccine. We included all patient ages and geographic locations and utilized 71 unique VAERS diagnostic codes pertaining to a hematologic condition as defined in the VAERS database. Fifty-five reports of hematologic events were identified (60.0% Pfizer-BioNTech, 27.3% Moderna, 7.3% Pfizer-BioNTech bivalent booster plus influenza, 5.5% Moderna bivalent booster plus influenza). The median age of patients was 66 years, and 90.9% (50/55) of reports involved a description of cytopenias or thrombosis. Notably, 3 potential cases of ITP and 1 case of VITT were identified. In one of the first safety analyses of the new SARS-CoV-2 booster vaccines, we identified few adverse hematologic events (1.05 per 1,000,000 doses), most of which could not be definitively attributed to vaccination. However, three reports of possible ITP and one report of possible VITT highlight the need for continued safety monitoring of these vaccines as their use expands and new formulations are authorized.

Decompressive surgery in cerebral venous sinus thrombosis due to vaccine-induced immune thrombotic thrombocytopenia.

BACKGROUND AND PURPOSE: Cerebral venous sinus thrombosis due to vaccine-induced immune thrombotic thrombocytopenia (CVST-VITT) is an adverse drug reaction occurring after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination. CVST-VITT patients often present with large intracerebral haemorrhages and a high proportion undergoes decompressive surgery. Clinical characteristics, therapeutic management and outcomes of CVST-VITT patients who underwent decompressive surgery are described and predictors of in-hospital mortality in these patients are explored. METHODS: Data from an ongoing international registry of patients who developed CVST within 28 days of SARS-CoV-2 vaccination, reported between 29 March 2021 and 10 May 2022, were used. Definite, probable and possible VITT cases, as defined by Pavord et al. (N Engl J Med 2021; 385: 1680-1689), were included. RESULTS: Decompressive surgery was performed in 34/128 (27%) patients with CVST-VITT. In-hospital mortality was 22/34 (65%) in the surgical and 27/94 (29%) in the non-surgical group (p < 0.001). In all surgical cases, the cause of death was brain herniation. The highest mortality rates were found amongst patients with preoperative coma (17/18, 94% vs. 4/14, 29% in the non-comatose; p < 0.001) and bilaterally absent pupillary reflexes (7/7, 100% vs. 6/9, 67% with unilaterally reactive pupil, and 4/11, 36% with bilaterally reactive pupils; p = 0.023). Postoperative imaging revealed worsening of index haemorrhagic lesion in 19 (70%) patients and new haemorrhagic lesions in 16 (59%) patients. At a median follow-up of 6 months, 8/10 of surgical CVST-VITT who survived admission were functionally independent. CONCLUSIONS: Almost two-thirds of surgical CVST-VITT patients died during hospital admission. Preoperative coma and bilateral absence of pupillary responses were associated with higher mortality rates. Survivors often achieved functional independence.

COVID-19 Vaccine-Associated Immune Thrombosis and Thrombocytopenia (VITT): Diagnostic Discrepancies and Global Implications.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) has been reported in association with the coronavirus disease 2019 preventative adenovirus vector-based vaccines ChAdOx1 nCoV-19 (Oxford/AstraZeneca) and Ad26.COV2.S (Janssen/Johnson & Johnson) in hundreds of recipients across the globe. VITT is characterized by thrombosis, typically at unusual sites, low fibrinogen, and elevated plasma D-dimer, generally manifesting between 4 and 28 days following vaccination. Detection of anti-platelet factor antibodies using an enzyme-linked immunosorbent assay (ELISA) is often confirmatory. Although several similar principles subside in most diagnostic criteria for VITT, the presentation of a positive ELISA assay, use of expert hematology and neurology opinion, and exclusion of possible VITT cases outside the "standard" 4 to 28-day timeframe have contributed a lack of global standardization for defining VITT. Accordingly, the global and regional incidence of VITT differs according to the diagnostic pathway and case definition used. This has influenced the public perception of VITT's severity and the decision to use adenovirus vector-based vaccines for limiting severe acute respiratory syndrome coronavirus 2 infection. We hereby delineate the recognized pathogenic mechanisms, global incidence, discrepancies in diagnostic criteria, recommended treatments, and global implications to vaccine hesitancy from this coagulopathy.

Quantitative interpretation of PF4/heparin-EIA optical densities in predicting platelet-activating VITT antibodies.

BACKGROUND: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a prothrombotic, heparin-induced thrombocytopenia (HIT)-mimicking, adverse reaction caused by platelet-activating anti-platelet factor 4 (PF4) antibodies that occurs rarely after adenovirus vector-based COVID-19 vaccination. Strength of PF4-dependent enzyme immunoassay (EIA) reactivity-judged by optical density (OD) measurements-strongly predicts platelet-activating properties of HIT antibodies in a functional test. Whether a similar relationship holds for VITT antibodies is unknown. OBJECTIVES: To evaluate probability for positive platelet activation testing for VITT antibodies based upon EIA OD reactivity; and to investigate simple approaches to minimize false-negative platelet activation testing for VITT. METHODS: All samples referred for VITT testing were systematically evaluated by semiquantitative in-house PF4/heparin-EIA (OD readings) and PF4-induced platelet activation (PIPA) testing within a cohort study. EIA-positive sera testing PIPA-negative were retested following 1/4 to 1/10 dilution. Logistic regression was performed to predict the probability of a positive PIPA per magnitude of EIA reactivity. RESULTS: Greater EIA ODs in sera from patients with suspected VITT correlated strongly with greater likelihood of PIPA reactivity. Of 61 sera (with OD values >1.0) testing negative in the PIPA, a high proportion (27/61, 44.3%) became PIPA positive when tested at 1/4 to 1/10 dilution. CONCLUSIONS: VITT serology resembles HIT in that greater EIA OD reactivity predicts higher probability of positive testing for platelet-activating antibodies. Unlike the situation with HIT antibodies, however, diluting putative VITT serum increases probability of a positive platelet activation assay, suggesting that optimal complex formation depends on the stoichiometric ratio of PF4 and anti-PF4 VITT antibodies.

NETosis and thrombosis in vaccine-induced immune thrombotic thrombocytopenia.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare yet serious adverse effect of the adenoviral vector vaccines ChAdOx1 nCoV-19 (AstraZeneca) and Ad26.COV2.S (Janssen) against COVID-19. The mechanisms involved in clot formation and thrombocytopenia in VITT are yet to be fully determined. Here we show neutrophils undergoing NETosis and confirm expression markers of NETs in VITT patients. VITT antibodies directly stimulate neutrophils to release NETs and induce thrombus formation containing abundant platelets, neutrophils, fibrin, extracellular DNA and citrullinated histone H3 in a flow microfluidics system and in vivo. Inhibition of NETosis prevents VITT-induced thrombosis in mice but not thrombocytopenia. In contrast, in vivo blockage of FcγRIIa abrogates both thrombosis and thrombocytopenia suggesting these are distinct processes. Our findings indicate that anti-PF4 antibodies activate blood cells via FcγRIIa and are responsible for thrombosis and thrombocytopenia in VITT. Future development of NETosis and FcγRIIa inhibitors are needed to treat VITT and similar immune thrombotic thrombocytopenia conditions more effectively, leading to better patient outcomes.

Second-dose ChAdOx1 and BNT162b2 COVID-19 vaccines and thrombocytopenic, thromboembolic and hemorrhagic events in Scotland.

We investigated thrombocytopenic, thromboembolic and hemorrhagic events following a second dose of ChAdOx1 and BNT162b2 using a self-controlled case series analysis. We used a national prospective cohort with 2.0 million(m) adults vaccinated with two doses of ChAdOx or 1.6 m with BNT162b2. The incidence rate ratio (IRR) for idiopathic thrombocytopenic purpura (ITP) 14-20 days post-ChAdOx1 second dose was 2.14, 95% confidence interval (CI) 0.90-5.08. The incidence of ITP post-second dose ChAdOx1 was 0.59 (0.37-0.89) per 100,000 doses. No evidence of an increased risk of CVST was found for the 0-27 day risk period (IRR 0.83, 95% CI 0.16 to 4.26). However, few (≤5) events arose within this risk period. It is perhaps noteworthy that these events all clustered in the 7-13 day period (IRR 4.06, 95% CI 0.94 to 17.51). No other associations were found for second dose ChAdOx1, or any association for second dose BNT162b2 vaccination. Second dose ChAdOx1 vaccination was associated with increased borderline risks of ITP and CVST events. However, these events were rare thus providing reassurance about the safety of these vaccines. Further analyses including more cases are required to determine more precisely the risk profile for ITP and CVST after a second dose of ChAdOx1 vaccine.

Relapse of immune thrombotic thrombocytopenic purpura (iTTP) possibly triggered by COVID-19 vaccination and/or concurrent COVID-19 infection.

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease that may be triggered by inflammation, including infection or vaccination. Since the start of the COVID-19 pandemic, several case reports were published on de novo or relapsed immune TTP (iTTP) in COVID-19-infected patients. Case reports of iTTP episodes following vaccination against COVID-19 are also emerging. We report a case of relapsed iTTP in a patient who received Moderna mRNA-1273 SARS-CoV-2 vaccine and developed concurrent severe COVID-19 infection. The patient's iTTP was successfully managed with caplacizumab, therapeutic plasma exchange and high-dose steroids. We summarise published cases of iTTP associated with COVID-19 infection or vaccination.