Diabetic immune dysfunction and severe influenza: A comprehensive narrative review with clinical implications

Diabetes mellitus is among the strongest independent risk factors for severe influenza, hospitalization, and influenza-associated acute respiratory distress syndrome (ARDS). The mechanistic basis of this association, and its clinical implications for immunomodulatory therapy, has been examined extensively in basic immunology and virology but is rarely synthesized for the practicing intensivist. This narrative review synthesizes the mechanistic basis by which diabetes predisposes to severe influenza and to translate the established pathogen-driven versus host-driven lung injury framework into a bedside-relevant heuristic for immunomodulatory decision-making. PubMed/MEDLINE and the Cochrane Library were searched from inception through April 2026, integrating evidence across innate and adaptive immunology, viral pathogenesis, and clinical trial data. The review was guided by the SANRA framework. Diabetes plausibly impairs early antiviral containment through disruption of type I interferon signaling, alveolar macrophage function, neutrophil-mediated barrier defense, and CD8⁺ T-cell responses, predominantly demonstrated in vitro and in murine models with supportive observational human data. The resulting prolonged viral replication, superimposed on baseline endothelial vulnerability, may contribute to rapid, severe lung injury. The framework applies most directly to type 2 diabetes; mechanistic differences between type 1 and type 2, and between acute glucose-driven and chronic structural mechanisms, are addressed. Available observational and meta-analytic evidence consistently associates corticosteroids with worse outcomes in influenza pneumonia. Selective adjunctive strategies, most notably mTOR inhibition with sirolimus and combination clarithromycin-naproxen-oseltamivir, show early hypothesis-generating signals but require confirmatory trials. This synthesis is intended as hypothesis-generating, not definitive.