Spike Protein at the Crossroads of Long-COVID and Vaccine-Induced Immune Thrombotic Thrombocytopenia Syndromes: A Cardio-Hematological Perspective

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein is central to viral infectivity, host–pathogen interactions, and immune recognition. Beyond its indispensable role in viral entry, accumulating experimental and clinical evidence indicates that spike protein may exert pleiotropic biological effects involving endothelial, cardiovascular, hematological, neurological, and immunological pathways. During natural infection, these effects occur in the context of active viral replication, additional viral antigens, and systemic inflammation, whereas vaccination induces a fundamentally different, transient exposure to a prefusion-stabilized spike antigen without viral propagation. In this review, we critically examine the molecular and clinicopathological properties of the SARS-CoV-2 spike protein and the biological differences between infection-derived and vaccine-derived spike exposure. Particular emphasis is placed on endothelial dysfunction, platelet and complement activation, immune-thrombosis, persistent viral antigens and tissue reservoirs in Long-COVID, and the distinct anti-PF4-mediated pathophysiology of vaccine-induced immune thrombotic thrombocytopenia (VITT). Emerging evidence suggests that persistent or dysregulated spike-related antigen exposure may contribute to chronic multisystem manifestations in a subset of individuals following SARS-CoV-2 infection; however, evidence linking persistent vaccine-derived spike to chronic clinical syndromes remains substantially more limited and does not currently establish causality. Integrating these observations, we propose the ‘Long-Spike’ hypothesis as a hypothesis-generating conceptual framework rather than a defined clinical syndrome. Further prospective studies integrating ultrasensitive antigen detection, tissue-based analyses, immunophenotyping, and cardiovascular and hematological biomarkers are required to determine the clinical relevance and causal significance of persistent spike-related antigens.