Understanding Mechanisms of Thrombosis and Thrombocytopenia with Adenoviral SARS-CoV-2 Vaccines: A Comprehensive Synopsis

Background: Thrombosis with thrombocytopenia syndrome (TTS) is a rare condition known to occur spontaneously or after heparin use. With the advent of COVID-19 vaccines during the pandemic, TTS cases emerged post-administration of adenoviral vaccines, termed vaccine-induced immune thrombosis and thrombocytopenia (VITT). In response, the TTS consortium was formed to deepen our understanding of this syndrome post-vaccination.

Methods: The consortium employed a comprehensive approach across five work-packages. This included designing cohort studies covering the entire English population and analysing local linked regional datasets to detect TTS occurrences in real-time. Various patient and healthy control specimens, including those from vaccinated individuals, underwent testing for anti-PF4 antibodies using three different assays. Patients who developed VITT after the AstraZeneca (AZD1222) COVID-19 vaccine underwent whole genome and RNA sequencing to identify genetic susceptibility factors. Multiple studies were conducted to investigate the mechanism of anti-PF4 antibody formation, including assessments of adenoviral vector structure and binding to PF4. Detailed studies were also conducted to understand the immune response to vaccines, the role of immune complexes involving PF4 and their effects on proinflammatory cytokines, neutrophil extracellular traps and platelets in TTS pathogenesis.

Results: Cohort studies revealed a higher risk of arterial and venous thrombosis after COVID-19 infection compared to vaccination. Specifically, regarding vaccines, the risk of thrombosis and/or thrombocytopenia was higher after the first dose of the AZD1222 vaccine but not with subsequent doses or mRNA vaccines. Regional linked data indicated that real-time ascertainment of diseases across multiple acute hospital sites' secure data environments is not yet feasible at scale. The overall background seroprevalence of anti-PF4 antibodies was low in healthy individuals, vaccinated individuals, and those infected with COVID-19. Whole genome sequencing did not identify significant variants predisposing to VITT, with ongoing work on RNA sequencing. An electrostatic interaction between the hexon hypervariable regions of the ChAdOx1 capsid and PF4 was suggested as a possible mechanism for anti-PF4 antibody development. Strong immune response drove the formation of neutrophil extracellular traps, significant inflammatory responses, and clot formation in distant organs. Platelet activation post-immune complex formation against PF4 was dependent on FcɣRIIa but independent of complement, also occurring through binding with c-Mpl. T-cell reactivity against the AZD1222 vaccine indicates potential cross-reactivity with prevalent human adenoviruses.

Conclusions: The consortium's comprehensive work has uncovered new potential mechanisms of VITT and identified novel biomarkers and therapeutic strategies for further development and validation. This is crucial as the combination of thrombosis and thrombocytopenia, alongside anti-PF4 antibodies, can occur without exposure to heparin or adenovirus vaccines.

Future considerations: Recommendations include the development of a national reference laboratory and registry for diagnosis and further study of TTS; future vaccine development using the adenoviral vector platform to focus on reduction of the electrostatic interaction between viral hexons and PF4; international genomics collaboration; and studies focussed on understanding the symptoms suffered by patients as well as strategies to ameliorate them.

Limitations: Direct identification of VITT patients was hindered by poor recording. The rarity of VITT limited the number of patients recruited for genomic and mechanistic studies.

Funding: This synopsis presents independent research funded by the National Institute for Health and Care Research (NIHR) Efficacy and Mechanism Evaluation (EME) programme as award number NIHR135073.