Comparative systemic pathogenicity and host tropism of ancestral SARS-CoV-2, Delta, and Omicron EG.5 variants in K18-hACE2 mice.

Introduction: SARS-CoV-2 undergoes persistent genomic evolution, yet prevalent circulating variants tend to evolve toward reduced pathogenicity. However, direct comparative pathogenesis data involving contemporary, immune-evasive strains-such as the Omicron EG.5 subvariant (a descendant of the globally dominant XBB lineage)-relative to ancestral strains remain scarce. Methods: To clarify the evolutionary trajectory of viral virulence, we conducted a systematic, head-to-head comparison of the in vivo pathogenicity of three pivotal strains: the ancestral Wild-type (Wuhan lineage), Delta, and the currently circulating Omicron EG.5. We utilized the highly susceptible K18-hACE2 transgenic mouse model to assess disease progression, viral burden, and histopathology. Results: Despite uniformly progressing to lethality in this model, the strains exhibited a distinct gradient in the kinetics and severity of disease. The Wild-type strain demonstrated the most rapid disease progression, highest viral loads across multiple organs (notably liver, brain, lung, and intestine), and most severe histopathological damage. The Delta variant showed an intermediate phenotype. Strikingly, the Omicron EG.5 variant displayed significantly attenuated pathogenicity, characterized by markedly delayed mortality, milder clinical presentation, and reduced tissue viral burden, yet it retained a capacity for multi-organ infection. Discussion: Our study provides direct experimental evidence for a pronounced attenuation in pathogenicity along the evolutionary trajectory from Wild-type through Delta to Omicron EG.5. These findings offer crucial insights for risk assessment of emerging variants and underscore the need for ongoing surveillance.